This is another article in the chapter Enormities from the classical manual on Sharia called Reliance of the Traveller. The articles are posted one by one in the order they are given in the book.
p9.0 LYING ABOUT THE PROPHET
p9.1 Some scholars hold that lying about the Prophet (Allah bless him and give him peace) is unbelief (kufr) that puts one beyond the pale of Islam. There is no doubt that a premeditated lie against Allah and His messenger that declares something which is unlawful to be permissible or something permissible to be unlawful is pure unbelief. The question (A: as to when it is an enormity rather than outright unbelief) only concerns lies about other than that.
p9.2 The Prophet (Allah bless him and give him peace) said:
(1) “A lie about me is not the same as a lie about someone else: whoever intentionally lies about me shall take a place for himself in hell.”
(2) “Whoever relates words purportedly from me, thinking it is a lie, is a liar.”
p9.3 It is clear from this that narrating a forged (mawdu’) hadith is not permissible.
p9.4 (Ibn Kathir:) As for detecting forged hadiths, there are many signs that enable one to do so, such as internal evidence of forgery in wording or content, including poor grammar, corrupt meaning, the mention of incredible rewards for inconsiderable efforts, or inconsistency with what is established in the Koran and rigorously authenticated (sahih) hadith. It is not permissible for anyone to relate such a hadith except by way of condemning it, to warn one of the ignorant public or common people who might be deceived by it.
There are many types of individuals who forge hadiths, including those with corrupt convictions about basic tenets of Islamic faith, as well as devotees who believe they are doing good by making up hadith-like stories that encourage others to do good, avoid bad, or perform meritorious acts, that such stories may be acted upon (al-Bahith alhathith sharh lkhtisar ‘ulum al-hadith (y61), 78).
p9.5 (n: Having discussed lies and forgeries, we must strictly distinguish them from the hadith category called not well authenticated (da’if, lit. “weak”), so termed because of such factors as having a channel of transmission containing a narrator whose memory was poor, one who was unreliable, unidentified by name, or for other reasons. Such hadiths legally differ from forgeries in the permissibility of ascribing them to the Prophet (Allah bless him and give him peace) and in other ways discussed at w48 below.)