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 was mentioned in the presence of the Prophet (peace and blessings be upon him), and another man praised him well. He (peace and blessings be upon him) said: "You have cut your brother's neck." He (peace and blessings be upon him) kept saying it repeatedly: "If one must praise, let him say: 'This is what I think of so-and-so.' If he is as he supposes him to be, then his reckoning rests with Allah; and let him not vouch for anyone's purity before Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21134