Riyad us Saliheen: Hadith 1788

Mention of a man was made to the Prophet (ﷺ) and someone praised him whereupon he (ﷺ) said, "Woe be to you! You have broken the neck of your friend!" He repeated this several times and added, "If one of you has to praise his friend at all, he should say: 'I reckon him to be such and such and Allah knows him well', if you think him to be so-and-so, you will be accountable to Allah because no one can testify the purity of others against Allah."

[Al-Bukhari and Muslim].