Abd al-Rahman bin Abi Bakrah (may Allah have mercy on him) narrates from his father, Sayyiduna Abu Bakrah (may Allah be pleased with him), that a man praised someone in the presence of the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) said several times: "You have cut your brother's neck. When one must praise, let him say: 'I think such of him, and Allah is the One who will hold him to account, and I do not vouch for anyone's purity before Allah; I think him to be such-and-such,' if he knows that about him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21135