Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 117
It is reported on the authority of Ibn 'Umar that the Messenger of Allah (may peace and blessings be upon him) said: Any person who called his brother: or unbeliever (has in fact done an act by which this unbelief) would return to one of them. If it were so, as he asserted (then the unbelief of man was confirmed but if it was not true), then it returned to him (to the man who labeled it on his brother Muslim).