From Sahih Bukhari (Hadith 7361):
Narrated Humaid bin 'Abdur-Rahman that he heard Mu'awiya talking to a group of people from Quraish at Al-Madina, and on mentioning Ka'b Al-Ashbar, he said, "He was one of the most truthful of those who used to talk about the people of the Scripture, yet we used to detect certain faults in his information
Reference: Bukhari 7361