Sahih Bukhari Volume 008, Book 073, Hadith Number 087.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 73. Good Manners and Form (Al-Adab)
Narated By Abu Bakra : A man was mentioned before the Prophet and another man praised him greatly The Prophet said, "May Allah's Mercy be on you ! You have cut the neck of your friend." The Prophet repeated this sentence many times and said, "If it is indispensable for anyone of you to praise someone, then he should say, 'I think that he is so-and-so," if he really thinks that he is such. Allah is the One Who will take his accounts (as He knows his reality) and no-one can sanctify anybody before Allah." (Khalid said, "Woe to you," instead of "Allah's Mercy be on you.")