Sahih Al Bukhari (655)

عن أبي بكرة قال {أثنى رجل على رجل عند النبي (صلى الله عليه وسلم) فقال ويلك قطعت عنق صاحبك قطعت عنق صاحبك مرارا ثم قال من كان منكم مادحا أخاه لا محالة فليقل أحسب فلانا والله حسيبه ولا أزكي على الله أحدا أحسبه كذا وكذا إن كان يعلم ذلك منه}

Abu Bakra narrated: A man praised another man in front of the Prophet (peace be upon him). The Prophet (peace be upon him) 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."