Musnad Ahmad 830:111

Mahbub ibn al-Hasan narrated to us, from Khalid, from Abd al-Rahman ibn Abi Bakrah, from his father, that a man praised a companion of his in the presence of the Prophet, peace and blessings of Allah be upon him. He said: "Woe to you, you have cut the neck of your companion! If you must praise him, then say: 'I reckon him to be such and such,' and Allah is his reckoner, and I do not purify anyone before Allah."