Good Manners and Form (Al-Adab)

Bukhari :: Book 8 :: Volume 73 :: Hadith 183

Narrated Abu Bakra:

A man praised another man in front of the Prophet. The Prophet said thrice, "Wailaka (Woe on you) ! You have cut the neck of your brother!" The Prophet added, "If it is indispensable for anyone of you to praise a person, then he should say, "I think that such-and-such person (is so-and-so), and Allah is the one who will take his accounts (as he knows his reality) and none can sanctify anybody before Allah (and that only if he knows well about that person.)".