Musnad Ahmad 830:139

Abu Bakrah said: A man was mentioned in the presence of the Prophet, may Allah bless him and grant him peace, and a man praised him highly. The Prophet of Allah, may Allah bless him and grant him peace, said: "Woe to you, you have cut the neck of your brother! By Allah, if he had heard it, he would never have succeeded." Then the Messenger of Allah, may Allah bless him and grant him peace, said: "If one of you must praise his brother, let him say: 'By Allah, I think such-and-such is so-and-so,' and I do not purify anyone before Allah."