The Messenger sallallaahu 'alayhi wa sallam said: "He who
says to his brother 'O Disbeliever', then it returns upon one of them."  He also said: "And he who accuses a believer
of Kufr (disbelief) then it is like killing him." 
Imaam an-Nawawee said, "They differ as regards interpretation
of this "returning", so it is said, "Kufr returns upon him if he is making that lawful", and this is far from the context
of the narration, and it is said, "it is taken to refer to the Khawaarij", since they declare believers to be Kaafiroon (disbelievers)."
Imaam al-Haafidh (Ibn Hajr) says, "And what is correct is that
the hadith (narration from the Messenger sallallaahu 'alayhi wa sallam was said as a warning against a Muslim saying that
to his brother. It is said, "What returns upon him is his speaking ill of his brother and the sin of declaring him a kaafir
(disbeliever)", and this is reasonable. It is also said, "It is to be feared that this will lead him into kufr", just as it
is said, "sins leads towards kufr". Thus it is feared that if he continues in that and persists in it then he will have a
bad end. I prefer from these sayings that it refers to the one who says it to someone from whom nothing is known except Islaam
and there is no justification or reason for him to claim that he is a kaafir. So in such a case he becomes a kaafir himself
because of that, and this will be explained. So the meaning of the hadith is that his judgement of takfeer (making someone
a kaafir) returns upon himself, so what is meant is takfeer not kufr. So it is as if he passed judgement of kuft upon himself
since he passed this judgement on one who is like him (ie. a muslim)"
Imaam ash-Shawkaanee said, "Judging that a Muslim has left
Islaam and entered into Kufr is something that is not fitting for a Muslim who believes in Allaah and the Last Day except
with a proof (Burhaan) which is clearer than the daytime sun, since it is established in the authentic hadith reported by
a group of Companions that he who says to his brother "O Kaafir!" Then it returns back to one of them." In another wording,
"Whoever addresses a man with Kufr, or says 'Enemy of Allaah' and he is not that, then it returns back upon him"
So in these ahadith (plural of hadith) and others similar to
them, there is the severest reprimand and the greatest warning against hurrying to perform takfeer. Allaah, The Mighty and
" ... and such as open their breasts to disbelief
... " [Soorah an-Nahl 16:106]
So what has to be the case is that the heart embraces kufr
and is at peace with it, and the soul is satisfied with it. So the appearance of wicked beliefs only is not to be taken into
account here, especially if one is ignorant of the fact that they are contrary to Islaam. Likewise account will not be taken
of the appearance of an action of kufr for the one who did not intend to leave Islaam by it and enter into kufr, and likewise
account will not be taken of a word spoken by a Muslim which is a saying of kufr, when he does not believe what it means."
So where do they stand those who hasten to perform takfeer
of the Muslims in relation to the saying of the Messenger sallallaahu 'alayhi wa sallam: "Abusing a Muslim is open sin
and killing him is kufr"  And in relation to his saying: "The Muslim is the Brother of a Muslim, he does not oppress
or forsake him."  Also in relation to his saying: "All of the Muslim to the Muslim is inviolable; his blood, his
wealth and his honour."  ?
Imaam Al-Qurtubee says in his tafseer (exegesis/explanation)
of Sooratul Hujuraat: " ... and His (Allaah) saying: "lest your deeds be rendered fruitless while you perceive it not'."
[Soorah al-Hujuraat 49:2] does not mean that a person becomes a kaafir without knowing since just as a person does not become
a believer except through choosing eemaan (faith) over kufr, then likewise a believer does not become a kaafir without intending
kufr and choosing it, and there is consensus (ijma') regarding this."
1. Reported by al-Bukhaaree, Muslim, Maalik, at-Tirmidhee and
2. Reported by al-Bukhaaree.
3. Abbreviated from an-Nawawee's "Sharh" (explanation) of Saheeh
4. Abbreviated from "Fathul-Baaree" (10/466)
5. Hadith reported by Muslim (2/49 - Book of Eemaan).
6. Taken from "as-Sailul-Jarraar" (4/578) and adapted.
7. Reported by al-Bukhaaree and Muslim.
8. Reported by Muslim.
9. Reported by Muslim, Abu Daawood and at-Tirmidhee.
10. Tafseer of al-Qurtubee (7/6128).