Weak Narration Concerning the prohibition of Helping to Kill a Muslim
Question

                There is a saheeh hadeeth (authentic narration) in which the Messenger of Allah (peace and blessings of Allah be upon him) said: “Whoever helps to kill a Muslim, even with half a word, will meet Allah with (the words) written between his eyes, ‘He has no hope of the mercy of Allah.’”– i.e., the mercy of Allah will not descend upon one who helps a kafir (non-Muslim) to kill a Muslim. This is what I understand from the hadeeth. Please explain further.
Praise be to Allah.
    Firstly:  

    This hadeeth was 
    narrated by Ibn Majah (2620) and al-‘Aqeeli in al-Du’afa (457) and 
    al-Bayhaqi in al-Sunan (8/22). It is a da’eef (weak) hadeeth which is not 
    soundly narrated from the Prophet (peace and blessings of Allah be upon 
    him). Imam Ahmad said: This hadeeth is not saheeh. 

    Indeed, Abu Hatim 
    ruled it to be mawdoo’ (fabricated), and al-Thahabi agreed with him. Ibn 
    al-Jawzi narrated it in al-Mawdoo’at (2/104). Ibn Hibban said: This is a 
    mawdoo’ (fabricated) hadeeth which has no basis. 

    Ibn Hajar and 
    al-Munthiri said: (This is) a very weak hadeeth. Al-Zayla’i said: it is a 
    weak (da’eef) hadeeth. 

    Al-Albani 
    mentioned it in Da’eef Sunan Ibn Majah and said: (it is) very weak. 

    Secondly: 

    Even though this 
    hadeeth is weak, there is no doubt that helping to kill a Muslim unlawfully 
    is a major sin. 

    Al-‘Izz ibn ‘Abd 
    al-Salam (may Allah have mercy on him) said in his book al-Qawa’id: 

    If you want to 
    know the difference between a minor sin and a major sin, then compare the 
    consequences of the sin with the consequences of a major sin as stated in 
    the texts. If it is less than the least consequences of a major sin, then it 
    is a minor sin. If it is equal to or greater than the least consequences of 
    a major sin, then it is also a major sin. Whoever insults the Lord, may He 
    be glorified, or His Messenger (peace and blessings of Allah be upon him), 
    or mocks the Messengers, or disbelieves in one of them, or throws filth at 
    the Ka’bah, or throws the Mus-haf (copy of the Quran)  into the garbage – 
    these are major sins, even though the sharee’ah does not state clearly that 
    they are major sins. Similarly, if a person holds down a chaste woman for 
    another to commit zina (adultery) with her, or holds down a Muslim for him 
    to kill him, the evil consequences of these acts are undoubtedly greater 
    than the consequences of consuming the wealth of an orphan, even though that 
    is also a major sin. Similarly if a person guides the kuffar (non-Muslims) 
    to the weak points of the Muslims, even though he knows that they will 
    destroy the Muslims as a result of his help and will take their women and 
    children captive, and plunder their wealth, then this is more serious than 
    his running away from the battle field with no excuse, even though that is a 
    major sin. Similarly if he tells a lie about a person for which he knows 
    that person will be killed, then it is a major sin, but telling a lie which 
    will result in a date being taken away from him, is not a major sin.”  

    Helping a kafir to 
    kill a Muslim may constitute kufr (disbelief) in some cases. The scholars 
    mentioned things that nullify a person’s Islam, one of which is supporting 
    the mushrikeen (polytheists) and helping them against the Muslims. See 
    question no. 
    
    33691. 

    Thirdly: 

    It is not 
    permissible for anyone to take the matter of attributing hadeeth to the 
    Prophet (peace and blessings of Allah be upon him) lightly. He (peace and 
    blessings of Allah be upon him) said: “Whoever tells lies about me, it is 
    not like telling lies about anyone else. Whoever tells lies about me 
    deliberately, let him take his place in Hell.” Narrated by al-Bukhari, 1291; 
    Muslim, 4. 

    And he (peace and 
    blessings of Allah be upon him) said: “Whoever narrates a hadeeth from me 
    which is known to be false, he is one of the liars.” Narrated by Muslim in 
    the introduction to his Saheeh. 

    And al-Nawawi 
    said: 

    [This hadeeth has 
    been narrated in two versions, one of which says al-kathibayn (the two liars 
    – dual form) and the more well-known version which says al-kathibeen (the 
    liars – plural).] Al-Qadi ‘Iyad said: The version which we have says 
    al-kathibeen (plural) and Abu Na’eem al-Asbahani narrated it in his book 
    al-Mustakhraj ‘ala Saheeh Muslim from the hadeeth of Samurah, where it says 
    al-kathibayn, in the dual form. Some of the imams said that the word yura 
    [translated above as “known to be” as yara [meaning “knowing (himself).” Or 
    it could mean “thinking [that it is false].” In this case the hadeeth means 
    that the person becomes a sinner only when he himself knows or thinks that 
    what he is narrating from the Prophet (peace and blessings of Allah be upon 
    him) is a lie. But if he did not know or think that it was a lie, then there 
    is no sin on him if he narrated it, even if others think that it is false, 
    or know that to be the case.  But what we may conclude from this hadeeth is 
    that lying is strongly condemned, and that whoever thinks that what he is 
    narrating is most likely false, but still narrates it, is a liar. How can he 
    not be a liar when he is narrating something that did not happen? 

    If a person does 
    not have sufficient knowledge to be able to distinguish between a saheeh 
    (authentic) hadeeth and a da’eef (weak) one, then he should ask the scholars 
    who are specialized in this field, and refer to the comments of the imams 
    and scholars that are compiled in books, in accordance with the words of 
    Allah (interpretation of the meaning): 

    “So ask of those who know the Scripture, if you know not.” [16:43].