“Ibn Umar related that the Holy Prophet said: If a Muslim calls another kafir, then if he is a kafir let it be so; otherwise, he [the caller] is himself a kafir.”
(Abu Dawud, Book of Sunna, edition published by Quran Mahal, Karachi, vol. iii, p. 484)
“Abu Zarr reported that the Holy Prophet said: No man accuses another man of being a sinner, or of being a kafir, but it reflects back on him if the other is not as he called him.”
(Bukhari, Book of Ethics; Book 78, ch. 44)
“Withhold [your tongues] from those who say 'There is no god but Allah’ — do not call them kafir. Whoever calls a reciter of 'There is no god but Allah’ as a kafir, is nearer to being a kafir himself.”
(Tabarani, reported from Ibn Umar)
“Call not the people of your Qibla [i.e. those who face the Ka'ba in Makka for prayer] as kafir.”
(Al-Nihaya of Ibn Athir, vol. iv, p. 187)
“Nothing expels a man from faith except the denial of that by which he entered into it [i.e. the Kalima].”
(Majma' az-Zawa’id, vol. i, p. 43)
“Three things are the basis of faith. [One is] to withhold from one who says 'There is no god but Allah’ — do not call him kafir for any sin, nor expel him from Islam for any misconduct.”
(Abu Dawud, Book of Jihad, 15:33)
“Whoever attributes kufr [unbelief] to a believer, he is like his murderer.”
(Tirmizi, ch. Iman (Faith); see Arabic-Urdu edition cited earlier, vol. ii, p. 213. See also Bukhari, Book of Ethics; Book 78, ch. 44)