In The Name of Allaah, The Most Merciful, The Bestower of Mercy

Narrated Sa’id bin Al-Musaiyab: ‘Umar came to the mosque while Hassaan was reciting a poem. (‘Umar disapproved of that). On that Hassaan said, “I used to recite poetry in this very mosque in the presence of one (i.e. the Prophet ) who was better than you.” Then he turned towards Abu Hurairah and said (to him), “I ask you by Allah, did you hear Allah’s Messenger saying (to me), ‘Retort on my behalf. O Allah! Support him (i.e., Hassaan) with the Ruh Al-Qudus [i.e. Jibril (Gabrael)]?” Abu Hurairah said, “Yes.” [Bukhaari 4/3212]

Imaam Abu Haatim [rahimahullaah] said: There is evidence in this narration similar to the command to disparage the du’afaa [weak narrators]. That is because the Prophet [sallal-laahu-alayhi-wasallam] said to Hassaan [radiyallaahu-anhu], ‘’Retort on my behalf.’’ He [sallal-laahu-alayhi-wasallam] commanded [Hassaan (radiyallaahu-anhu)] to defend him against the lies that the idol worshippers uttered about him. And when it is the case that the Messenger of Allaah [sallal-laahu-alayhi-wasallam] commanded [Hassaan] to defend him against the lies of the idol worshippers, even though neither did their lies harm the Muslims nor make what is Haraam to be Halaal and vice versa; then it is more appropriate that he [sallal-laahu-alayhi-wasallam] is defended against those Muslims who lie about him [sallal-laahu-alahu-wasallam] through their [weak, fabricated] narrations and thus declare Haraam as Halaal and vice versa. [1]

[Source: Muqaddimah Kitaab Al-Majrooheem Minal Muhadditheen of Imaam Ibn Hibbaan (rahimahullaah). Page: 67-68. Slightly paraphrased]

Ref 1: Notes: Must Read: Regarding the Affair of Declaring Something Halaal that is Haraam and vice versa: http://www.spubs.com/sps/sp.cfm?subsecID=GSC05&articleID=GSC050006&pfriend=

Also see article about Takfeer on these links:

http://www.salafipublications.com/sps/sp.cfm?secID=MNJ&subsecID=MNJ09&loadpage=displaysubsection.cfm

http://www.manhaj.com/manhaj/articles/xwouo-takfir-and-the-excuse-of-ignorance-shaykh-ibn-uthaymeen.cfm