A questioner is commenting on the previous answer about fasting and the hadeeth of Ibn ‘Abbaas about moon sighting with one witness
Question

                In your answer to question number 26824, you stated that it is permissible to accept the opinion of a trustworthy person with regard to moon sighting, but this contradicts the hadeeth that says that a Bedouin came to the Messenger (blessings and peace of Allah be upon him) and told him that he had seen the moon. When the Messenger asked him, “Do you believe that there is no god but Allah and that Muhammad is the Messenger of Allah?” and he answered in the affirmative, he asked him: “Do you bear witness that you have seen the moon?” Hence this hadeeth is evidence that it is permissible to accept the moon sighting from any Muslim.
Praise be to Allah.
    The hadeeth that the questioner is referring to is: 

    It was narrated that Ibn ‘Abbaas (may Allah be pleased with 
    him) said: A Bedouin came to the Prophet (blessings and peace of Allah be 
    upon him) and said, I have seen the new moon tonight. He said, “Do you bear 
    witness that there is no god except Allaah and that Muhammad is the 
    Messenger of Allaah (blessings and peace of Allah be upon him)?” He said, 
    Yes. He said, “Get up, O Bilaal, and announce to the people that they should 
    fast tomorrow.” 

    Narrated by al-Tirmidhi (691), Abu Dawood (2340), al-Nasaa’i 
    (2112) and Ibn Maajah (1652). The hadeeth is da’eef (weak) and not saheeh. 
    It was classed as weak by al-Nasaa’i, al-Albaani and others. 

    As the hadeeth is weak, there is no contradiction between it 
    and what we have mentioned about it being essential that the one who sights 
    the moon should be of good character. 

    Even if we assume that the hadeeth is saheeh, it may be 
    interpreted in several ways, such as: 

    1.

    That the issue with regard to accepting the testimony of the 
    one who has seen the new moon and the issue of determining whether he is 
    trustworthy and of good character is up to the judge to decide. If he is 
    confident, because of his experience with people, that this person who has 
    seen the moon is trustworthy in his testimony, he may accept this testimony 
    from him, even if no one knows him and is able to confirm that he is of good 
    character and trustworthy. 

    Shaykh al-Albaani (may Allah have mercy on him) said: 

    So we see from the hadeeth that he instructed Bilaal to 
    announce to the people that they would fast the next day. So the Messenger 
    (blessings and peace of Allah be upon him) was content with the testimony of 
    this man, whom he did not know, on the basis that he bore witness that there 
    is no God but Allah and that Muhammad is the Messenger of Allah, i.e. he 
    knew that he was a Muslim, but he did not check on him any further and he 
    did not try to find out how intelligent and smart he was, as was the case in 
    the first hadeeth in which the witness was ‘Abd-Allah ibn ‘Umar ibn 
    al-Khattaab. Yet despite that he accepted his testimony. This hadeeth makes 
    things easier for people, and what this means is that the judge should be 
    content with the witness as he appears to be, without needing to find people 
    who know this man and can testify that he is of good character, as was the 
    habit of judges since time immemorial. Rather it is sufficient to know that 
    he is a Muslim. This man was a Bedouin of whom the Prophet (blessings and 
    peace of Allah be upon him) had no prior knowledge and he was content that 
    he uttered the Shahaadatayn before him. So he was a Muslim with the same 
    rights and duties as any other, and based on his testimony and the fact that 
    he was a Muslim he said: O Bilaal, announced to the people that they should 
    fast tomorrow. 

    Al-Ta’laaq ‘ala Kitaab Buloogh al-Maraam, 
    (audio tape), hadeeth 5, Kitaab al-Siyaam. 

    2.

    That this hadeeth is evidence for the principle that a Muslim 
    is to be regarded as being of good character unless proven otherwise. 
    Al-San’aani (may Allah have mercy on him) said concerning what we learn from 
    the hadeeth of Ibn ‘Abbaas: 

    It indicates that the basic principle with regard to the 
    Muslims is that they are of good character, because the Prophet (blessings 
    and peace of Allah be upon him) did not ask the Bedouin for anything except 
    the Shahaadah. 

    Subul al-Salaam by al-San’aani, 
    2/153 

    3.

    That this ruling only applies to the Sahaabah, which is the 
    case because they are all to be regarded as being of good character. There 
    is no doubt that that Bedouin is to be included as one of the Sahaabah (may 
    Allah be pleased with them), and thus he is to be regarded as a man of good 
    character, one of those who do not need to be examined to prove that they 
    are of good character. 

    Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have 
    mercy on him) said: 

    All of the Sahaabah are trustworthy and of good character, 
    and the narration of any of them is to be accepted even if he is not known 
    by name. Hence they said: Not knowing the name of the Sahaabi does not 
    affect the validity of the hadeeth. 

    The evidence for what we have said about the Sahaabah is that 
    Allah and His Messenger praised them in a number of texts, and the Prophet 
    (blessings and peace of Allah be upon him) would accept the testimony of any 
    of them once he knew that he was Muslim, and he would not enquire any 
    further about him. It was narrated that Ibn ‘Abbaas (may Allah be pleased 
    with him) said: A Bedouin came to the Prophet (blessings and peace of Allah 
    be upon him) and said: I have seen the new moon, meaning Ramadan. 

    End quote from Mustalah al-Hadeeth, from his website. 

    There is something else that supports what we have stated 
    above, which is the fact that this testimony came at the time of Revelation, 
    and the testimony of the Bedouin could not have been accepted if it was a 
    false testimony concerning one of the acts of worship of the Muslims. 

    But as the hadeeth is da’eef (weak) there is no need to 
    discuss its meaning. Praise be to Allah the Lord of the Worlds. 

    And Allah knows best.