Sahih Muslim Book 1, Hadith Number 278.

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 reported: The Messenger of Allah (may peace be upon him) bestowed upon a group of persons (booty) and I was sitting with them. The remaining part of the hadith is the same as mentioned (above) with the addition I stood up and went to the Messenger of Allah (may peace be upon him) and whispered to him: Why did you omit such and such a man?