Are ahaad hadiths probably sound or definitively sound, and what is the view of at-Tabari and Ibn Taymiyah concerning that?
Question

Yesterday I had a discussion with someone who rejects the Sunnah. He told me that at-Tabari and Ibn Taymiyah (may Allah have mercy on them both) said that hadiths are probably soundly narrated from the Prophet (blessings and peace of Allah be upon him) but are not definitively so. Among his specious arguments he said that ahaad hadiths are not definitively sound; rather they are probably sound. I hope that you can explain this specious argument and refute it.
Praise be to Allah.1. Rejecting ahaad hadiths is rejecting most of what is narrated from the Prophet (blessings and peace of Allah be upon him) [as-Sunnah an-nabawiyyah]
If a hadith is soundly narrated from the Messenger of Allah (blessings and peace of Allah be upon him), we must accept it and act in accordance with it, without differentiating between ahaad hadiths and mutawaatir hadiths.
This is the view of the Sahaabah and Taabi‘een, and of those who followed them in truth. They accept the hadith if its isnaad is sound, and they act in accordance with it, and they do not hesitate to do that.
The one who rejects ahaad hadiths rejects most of the Sunnah, because most of the reports are ahaad hadiths. This is contrary to the command of Allah, may He be exalted (interpretation of the meaning): {And whatever the Messenger has given you - take; and what he has forbidden you - refrain from} [al-Hashr 59:7].
This is a Qur’anic injunction to accept and follow what is narrated from the Messenger (blessings and peace of Allah be upon him), and it cannot be understood as referring only to the mutawaatir reports, which are few in number and limited, and do not cover all beliefs, acts of worship, interactions, and morals and manners.
The call to reject ahaad reports is a call to reject most of the Sunnah and to ignore the command mentioned in this verse.
2. What is meant by hadiths that are definitively proven
What is meant by hadiths that are definitively proven is hadiths that can be definitively proven to have been soundly narrated from the Messenger (blessings and peace of Allah be upon him). They are of four types:

Mutawaatir hadiths.
Those which were narrated by al-Bukhaari and Muslim and were accepted by the ummah.
Those which the ummah unanimously agreed on and accepted as sound, even though they are not narrated in as-Saheehayn, because of what is well established, that the ummah is protected from error when they reach consensus on something.
Widely-known reports that are narrated through many different chains of narrators, none of which are subject to any criticism.

For an explanation of that, please see the answer to question no. 197164 .
Based on that, the claim that all the ahaad hadiths are not definitively proven is not correct; rather the ahaad hadiths that the ummah has accepted and regarded as sound, or that are widely circulated and narrated through many chains of narrators but have not reached the level of being mutawaatir, are still to be regarded as definitively proven.
3. Dividing hadiths into mutawaatir and ahaad does not mean that there is doubt about the ahaad hadiths
In the answer to question no. 126571, we explained that dividing hadiths into mutawaatir and ahaad does not mean that there is doubt about the ahaad hadiths, and regarding the mutawaatir hadiths as being definitively sound does not mean that we cannot rely on the ahaad hadiths and accept them. We gave an example of that and explained it, so please refer to that question.
4. Ahl as-Sunnah are unanimously agreed on accepting ahaad hadiths with regard to beliefs
Ahl as-Sunnah are unanimously agreed on accepting ahaad hadiths with regard to beliefs even if the hadith is to be attributed to the Prophet (blessings and peace of Allah be upon him) on the basis of probability. No one disagreed with that except innovators such as the Mu‘tazilah and their ilk.
Imam Ibn ‘Abd al-Barr (may Allah have mercy on him) said: Our companions and others differed regarding a report narrated by one narrator of good character: does it mean that it is definitively sound and we should act in accordance with it, or does it mean only that we should act in accordance with it but it is not definitively sound?
The view of most of the scholars regarding a report narrated by a sole narrator is that we should act in accordance with it but we cannot say that it is definitively sound [rather it is probably sound]. This is the view of ash-Shaafa‘i and most of the scholars of fiqh and men of reason; according to their view, we cannot be absolutely certain unless there is definitive proof, and there is no difference of opinion concerning that.
Many scholars of hadith, and some men of reason – including al-Husayn al-Karaabeesi and others – said that we should both be certain of it and act in accordance with it. Ibn Khuwayz Mindaad stated that this view is in accordance with the view of Maalik.
Abu ‘Umar said: What we say is that we should act in accordance with it but we cannot be completely certain of it, like when you have witnesses, whether two or four. This is the view of most of the scholars of fiqh and hadith; all of them are of the view that ahaad hadiths are to be accepted regarding issues of belief. This is the view of a number of Ahl as-Sunnah.. And Allah is our source of strength." (At-Tamheed  1/7).
And he (may Allah have mercy on him) said: The scholars of fiqh and hadith in all regions, as far as I know, are unanimously agreed that the report of a single narrator of good character is to be accepted, and it is obligatory to act in accordance with it if it is proven and not abrogated by another report or by scholarly consensus.
This is the view of all the fuqaha’ throughout the ages, from the time of the Sahaabah until our own time, except the Khaarijis and some small groups of innovators who do not have any impact on scholarly consensus." (At-Tamheed  1/2).
So whether it is said that ahaad reports are proven on the basis of probability or are proven definitively, they must be accepted and acted upon with regard to both rulings and beliefs.
5. The view of Imam at-Tabari (may Allah have mercy on him) regarding ahaad hadiths
The view of Imam at-Tabari (may Allah have mercy on him) does not differ from that of Ibn ‘Abd al-Barr, for he thinks that ahaad hadiths are not definitively proven, but we should accept them and act in accordance with them, even with regard to beliefs. He (may Allah have mercy on him) followed this view in his books.
He (may Allah have mercy on him) said: If the report is a kind that is as definitive as one who himself witnessed and heard, then the one who heard it should testify that what he is reporting is what happened, like testifying to a fact that he has seen and heard.
But if the report is a kind that is not definitive and does not dispel all doubts, but the one who is transmitting it is a person who is honest and of good character, then the one who hears his report must believe him and testify that he is narrating it as he heard it from him. That is similar to what we say about reports narrated by individuals of good character. We have explained this elsewhere, and there is no need to repeat it here." (At-Tabseer fi Ma‘aalim ad-Deen  p. 139).
And he (may Allah have mercy on him) said: That is because the one whom the report reaches via one who heard it from the Messenger (blessings and peace of Allah be upon him) can only be one of two things:

Either the one who narrated the report to him is one individual or a group who are like one individual, meaning that they conveyed it in such a way that they did not leave him with no excuse not to follow it.
Or he received the report from a group in such a way that he was left with no excuse not to follow it.

So if the one who conveyed that to him was one individual, or a group who are like one individual, in the sense that they would not leave no excuse for the one to whom they conveyed it not to accept it, then in that case:
If there is no one among them who is of good character and truthful, he does not have to act in accordance with the report and he does not have to accept their report.
If there is one among them who is of good character and truthful, then if that narrator conveyed the report to him, he must act in accordance with it but he does not have to believe that it is definitive." (Tahdheeb al-Aathaar  2/768).
And he (may Allah have mercy on him) said: There are some reports narrated by one narrator of good character or a group whose reporting does not lead to believing that their report is definitively sound, and does not leave the one who hears it with no excuse for not accepting it, but the one who hears it must still accept it.
There are some reports which are narrated in such a way that makes it obligatory for the one who hears them to believe that they are definitively proven, and leave no excuse for the one who hears them not to accept them. This is the transmission of a report by a group of people who are not prone to forgetfulness and error, and the way in which the report is transmitted shows that it is not possible that they could all have agreed upon a lie."(Tahdheeb al-Athaar, Musnad Talhah wa’-Zubayr, p. 439.
And he (may Allah have mercy on him) said: There is no report that I have narrated or not narrated, with a sound chain of narrators who are trustworthy and of good character, going back to the Messenger of Allah (blessings and of Allah be upon him), but for us it is a true and sound report, and the Muslims are bound to accept it and act in accordance with it." (Tahdheeb al-Athaar  2/713).
Thus he (may Allah have mercy on him) regards it as obligatory to act in accordance with and accept as sound ahaad reports, and the ummah should accept them and act in accordance with them without differentiating between whether the report explains rulings or otherwise.
See: Usool ad-Deen ‘inda al-Imam at-Tabari by Dr. Ta-Ha Muhammad Naja, p. 61; Manhaj al-Imam Ibn Jareer at-Tabari fi Naqd al-Ahaadeeth, by Dr. Nabeelah Zayd ibn Sa‘d (2/707).
6. The view of Ibn Taymiyah on acting in accordance with ahaad hadiths
With regard to Ibn Taymiyah, he thought that if there is evidence to prove that the ahaad hadith is sound, then we should believe that it is definitively proven, and he quoted ahaad reports as evidence for both rulings and beliefs.
He (may Allah have mercy on him) said: Hence the correct view is that the ahaad report (khabar waahid) could reach the point of being definitively proven, if there is other evidence to raise it to the level of being definitive. Based on that, many of the texts in as-Saheehayn are regarded as being mutawaatir by scholars of hadith, even if others do not realize that they are mutawaatir. Hence most of the texts of as-Saheehayn are the type of reports which the scholars of hadith know definitively that the Prophet (blessings and peace of Allah be upon him) said those words, sometimes because they are mutawaatir in their view, and sometimes because the ummah accepted them and believed in their soundness.
The ahaad report (khabar waahid) that is widely accepted means that the hadith is definitively proven according to the majority of scholars among the companions of Abu Haneefah, Maalik, ash-Shaafa‘i and Ahmad. This is the view of most of the companions of al-Ash‘ari, such as al-Isfaraayeeni and Ibn Foorak. Although if we examine the report in and of itself, we would think that the Prophet (blessings and peace of Allah be upon him) said it on the basis of probability, when there is scholarly consensus on accepting the hadith and believing it to be sound, it becomes like when the scholars of fiqh agree unanimously on a certain ruling, basing that ruling on what is apparent, or analogy, or an ahaad report (khabar waahid). Thus that ruling becomes definitive according to the majority, even though without taking this consensus into account it is not definitive, because the consensus of the ummah is protected from error.
The scholars who explain shar‘i rulings never agree unanimously to make permissible something that is prohibited or to make prohibited something that is permissible. The same applies to the scholars of hadith: they never agree unanimously to accept a fabricated hadith or to reject a sound hadith.
Sometimes one of them is aware of other, corroborating evidence to support a particular hadith, which makes him believe that it is definitively sound, and whoever comes to know what he knows about that hadith will reach the same conclusion as he reached." (Majmoo‘ al-Fataawa 18/40-41).
Conclusion:
All of the Muslim scholars accept ahaad hadiths concerning rulings on what is halaal and haraam. Most of them – and this is the view of Ahl as-Sunnah – also accept ahaad hadiths regarding beliefs. The issue of whether the reports are proven on the basis of probability or are definitive does not affect the obligation to accept them and act in accordance with them.
Regarding the ruling on one who rejects the Sunnah, please see the answer to question no. 115125 .
And Allah knows best.