Sahih Bukhari Volume 3, Book 48, Hadith Number 830.

Narated By Abu Bakra : A man praised another man in front of the Prophet. The Prophet said to him, “Woe to you, you have cut off your companion’s neck, you have cut off your companion’s neck,” repeating it several times and then added, “Whoever amongst you has to praise his brother should say, ‘I think that he is so and so, and Allah knows exactly the truth, and I do not confirm anybody’s good conduct before Allah, but I think him so and so,’ if he really knows what he says about him.”