Sahih Muslim Book 1, Hadith Number 276.

Chapter : To habituate the heart with Faith which due to its weakness, is scared and the forbiddance to call positively one believer without a definite proof.

Sa’d narrated it on the authority of his father (Abi Waqqas) that he observed: The Messenger of Allah (may peace be upon him) distributed shares (of booty among his Companions). I said: Messenger of Allah! Give it to so and so, for verily he is a believer. Upon this the Apostle of Allah remarked: Or a Muslim. I (the narrator) repeated it (the word “believer”) thrice and he (the Holy Prophet) turned his back upon me (and substituted the word) “Muslim,” and then observed: I bestow it (this share) to a man out of apprehension lest Allah should throw him prostrate into the fire (of Hell) whereas in fact the other man is dearer to me than he.