“It has been reported in a hadeeth that the Prophet, may peace and blessings be upon him, said: “Whosoever declares a Muslim to be an unbeliever has indeed disbelieved.” So if a person declares a Muslim to be an unbeliever, does that person become a disbeliever and apostate himself? Or is it that this hadeeth only points out the great evil of declaring a Muslim to be an unbeliever?”
“I am not aware of its authenticity right now, however its meaning is correct, for indeed the Prophet, may peace and blessings be upon him said:
“Whoever says to his brother “O you enemy of Allah” or says “O you Kafir (disbeliever)”, then one of them will deserve the title.” [Al-Bukharee and Muslim]
Meaning that if the one addressed is not deserving of it, the statement will return to the one who (initially) said it. Therefore it is impermissible for a Muslim to declare his fellow Muslim brother to be an unbeliever, nor to say ‘O you enemy of Allah’ and to say ‘O you Fajir’ (wicked evil doer), except with proof.
The one who accuses his brother of being an unbeliever without him being one, then his speech will return to him. The meaning of that is that it is a warning and not that it is major Kufr (disbelief). Rather the meaning of that is that it is a warning against this evil speech and that the one who says it is in great danger if he says it to his brother. So it is therefore befitting that one safeguards the tongue and that one does not speak except with knowledge.”
The presenter: “May Allah reward You with good and benefit (the people) with your knowledge.”
Shaykh ‘Abdul-‘Azeez bin ‘Abdullah bin Baaz, may Allah have mercy upon him.
Translated by Salafi-Dawah.com
source : http://www.salafi-dawah.com/explaining-whoever-says-to-his-brother-o-you-kafir.html