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.
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.
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].