The hadith “Woe to ‘Ammaar; he will be killed by the transgressing group; he will be calling them to Paradise and they will be calling him to Hell,” and refutation of the specious argument which quotes it to cast aspersions upon Mu‘aawiyah
Question

Some of the Raafidis quote the hadith “Woe to ‘Ammaar; he will be killed by the transgressing group; he will be calling them to Paradise and they will be calling him to Hell” which appears in al-Bukhaari, and there is, frankly, an argument for them in this hadith to support their claim that Mu‘aawiyah (may Allah be pleased with him) was a kaafir, and there are not many answers to this flawed argument on the Internet.
I hope that you can highlight the flaws in this argument.
Praise be to Allah.Firstly:
What is required is to think positively of those whom Allah chose to accompany His Prophet (blessings and peace of Allah be upon him): for they are the best companions of the best Prophet, and it is not permissible to say anything of them but the highest words of praise; if anyone criticises them, then this gives rise to suspicion about the sincerity of his religious commitment.
Abu Zar‘ah ar-Raazi (may Allah have mercy on him) said: If you see a man criticising any of the Companions of the Messenger of Allah (blessings and peace of Allah be upon him), then know that he is a heretic. That is because in our view, the Messenger of Allah (blessings and peace of Allah be upon him) is true, and the Qur’an is true. No one transmitted this Qur’an and the Sunnah to us except the Companions of the Messenger of Allah (blessings and peace of Allah be upon him). They only seek to undermine our witnesses (to the authenticity of the texts) in order to declare the Qur’an and Sunnah false, but these people are more deserving of criticism, for they are heretics.
End quote from al-Kifaayah fi ‘Ilm ar-Riwaayah by al-Khateeb al-Baghdadi (p. 49).
See also the answer to question no.  187689
Secondly:
The Companions of the Prophet (blessings and peace of Allah be upon him) fought one another ,and each party had a reason to fight, which was based on their own understanding of the texts and on their own ijtihaad, whereby each party believed it was in the right. Hence when it became clear to some of them that they were mistaken, they regretted going out and fighting, and regret is repentance, and repentance erases what comes before it, especially in the case of the noblest of mankind, and the highest in status and most respected of them after the Prophets and Messengers of Allah.
Whoever examines this matter properly will realise that this fighting was caused by those who sought to spread discord by spreading rumours and false ideas.
Many of the Sahaabah (may Allah be pleased with them) went out during this fighting seeking to reconcile between the people. Fighting was the most hateful thing to them, but the decree of Allah inevitably came to pass.
Thirdly:
Al-Bukhaari (447) narrated from Abu Sa ‘eed al-Khudri, in the report about the building of the mosque (in Madinah), that he said: We would carry one brick at a time, but  ‘Ammaar would carry two bricks at a time. The Prophet (blessings and peace of Allah be upon him) saw him and brushed the dust from him, saying: “Woe to ‘Ammaar; he will be killed by the transgressing group; he will be calling them to Paradise and they will be calling him to Hell”. ‘Ammaar said: I seek refuge with Allah from tribulation.
What is meant by calling to Paradise in this hadith is calling to the means of attaining it, which is obedience to the caliph (Ameer al-Mu’mineen), and what is meant by calling to Hell is calling to that which leads to it, which is opposing the caliph and rebelling against him.
But whoever did that on the basis of ijtihaad and valid interpretation is excused.
Al-Haafiz Ibn Katheer (may Allah have mercy on him) said:
This hadith is one of the signs of Prophethood, as the Prophet (blessings and peace of Allah be upon him) foretold to ‘Ammaar that he would be killed by the transgressing group, and he was killed by the people of ash-Shaam (greater Syria) in the battle of Siffeen, when ‘Ammaar was with ‘Ali  and the people of Iraq, as will be explained in detail below. ‘Ali had more right to rulership than Mu‘aawiyah did.
The fact that the companions of Mu‘aawiyah are described as transgressors does not imply that they were disbelievers, as the ignorant followers of the misguided sects, such as the Shi‘ah  and others, try to claim. That is because their decision to fight was based on their own ijtihaad and interpretation of the texts, and not everyone who engages in ijtihaad gets its right; rather the one who gets it right will have a twofold reward and the one who gets it wrong will have a single reward.
Those who added to this hadith after the phrase “you will be killed by the transgressing group” the words “Allah will not grant them my intercession on the Day of Resurrection” fabricated this additional material, falsely attributing it to the Messenger of Allah (blessings and peace of Allah be upon him), for he did not say it, as it was not narrated via any acceptable chain of narration. And Allah knows best.
With regard to the words “he will be calling them to Paradise and they will be calling him to Hell”, ‘Ammaar and his companions were calling the people of greater Syria to reconciliation and unity, whereas the people of greater Syria wanted to seize power from one who was more entitled to it, and they wanted the people to be divided, with each part of the Muslim lands having its own ruler, which would lead to disunity and division of the ummah, for this was the implication of their view and the consequences to which it would lead, even if they did not intend that. And Allah knows best.
End quote from al-Bidaayah wa’n-Nihaayah (4/538)
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
If it is said: “He was killed at Siffeen, when he was with ‘Ali, and those who killed him were with Mu‘aawiyah, with whom were some of the Sahaabah, so how can it be said that they were calling to Hell?”
The answer to that is that they thought that they were calling to Paradise, and their action was based on ijtihaad, so there is no blame on them for following what they thought was best. What is meant by calling people to Paradise is calling them to that which leads to it, which is obeying the ruler. This was the stance of ‘Ammaar, who was calling them to obey ‘Ali, who was the ruler whom it was obligatory to obey at that time, whereas they (Mu‘aawiyah and his followers) were calling people to something other than that, but they are excused for the wrong conclusion they reached, which was based on misinterpretation of the texts.
End quote from Fath al-Baari (1/542). See also: Majmoo‘ Fatawa Shaykh al-Islam (4/437)
So it is essential to differentiate between the one who tries to work it out but reaches a wrong conclusion, and the one who deliberately causes mischief and turmoil.
Allah, may He be exalted, says (interpretation of the meaning):
“And if two parties or groups among the believers fall to fighting, then make peace between them both, but if one of them rebels against the other, then fight you (all) against the one that which rebels till it complies with the Command of Allah; then if it complies, then make reconciliation between them justly, and be equitable. Verily! Allah loves those who are equitable.
The believers are nothing else than brothers (in Islamic religion). So make reconciliation between your brothers, and fear Allah, that you may receive mercy”
[al-Hujuraat 49:9-10].
This indicates that it is possible for fighting to occur among the believers, without that meaning that one of the groups is regarded as no longer being believers because of their fighting the other group. Then Allah, may He be exalted, says: “The believers are nothing else than brothers (in Islamic religion). So make reconciliation between your brothers” [al-Hujuraat 49:9-10].
So Allah described them as being brothers, despite their fighting, and He commanded the Muslims to reconcile between them.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Allah, may He be glorified and exalted, explains that despite their fighting and transgression against one another, they are still believers and brothers, and He enjoined reconciling between them. But if one of them transgresses after that, then the transgressing group is to be fought, but He did not enjoin fighting from the outset.
The Prophet (blessings and peace of Allah be upon him) stated that the groups that would pass out of the faith [the Khawaarij or Khaarijis] would be killed by the closer of the two groups to the truth, and ‘Ali ibn Abi Taalib and those who were with him were the ones who fought them.
The words of the Prophet (blessings and peace of Allah be upon him) indicate that they were closer to the truth than Mu‘aawiyah and those who were with him, although both groups were believers.
End quote from Majmoo‘ al-Fatawa (25/305-306)
It was narrated that Abu Sa‘eed al-Khudri said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “A group will secede from my ummah at a time of division among the Muslims, and they will be killed by the group that is closer to the truth.”
Narrated by Muslim (1064).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
This saheeh hadeeth indicates that both of the groups who engaged in fighting – ‘Ali and his companions, and Mu‘aawiyah and his companions – were following the truth, and that ‘Ali and his companions were closer to the truth than Mu‘aawiyah and his companions.
End quote from Majmoo‘ al-Fatawa (4/467). See also: Majmoo‘ al-Fatawa (4/437-438).
To sum up:
The words “they will be calling him to Hell” do not imply disbelief – Allah forbid. The one who says that is only highlighting his extreme ignorance. Rather these hadiths come under the heading of the hadiths of warning, like those hadiths which say that the one who consumes riba will be in the fire, and the one who consumes the orphan’s wealth will be in the fire, and other hadiths that contain warnings; they do not necessarily imply that the person who does that is a disbeliever, although they do indicate that his deed is haraam and is in fact a major sin.
In fact some of the scholars interpreted the words “they will be calling him to Hell” as referring to the Khawaarij (Kharijites).
Ibn Battaal (may Allah have mercy on him) said:
The words “he will be calling them to Paradise and they will be calling him to Hell” can only be correctly applied to the Khawaarij to whom ‘Ali sent ‘Ammaar to call them to join the jamaa‘ah (main body of the Muslims); they cannot be correctly applied to any of the Sahaabah, because it is not possible for any of the Muslims to interpret any of their actions except in the best manner, because they were the companions of the Messenger of Allah (blessings and peace of Allah be upon him), whom Allah praised and testified to their virtue, and said (interpretation of the meaning): “You  are the best of peoples ever raised up for mankind” [Aal ‘Imraan 3:110].
The commentators said: This refers to the Companions of the Messenger of Allah. It is narrated in a saheeh report that ‘Ali sent ‘Ammaar to the Khawaarij to call them to join the jamaa‘ah (main body of the Muslims), with whom there is protection from error.
End quote from Sharh Saheeh al-Bukhaari (2/98-99)
And Allah knows best.