ENOfficial
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."
Source: sunnah.com
Other translations
Arabic
وَقَالَ أَبُو الْيَمَانِ أَخْبَرَنَا شُعَيْبٌ، عَنِ الزُّهْرِيِّ، أَخْبَرَنِي حُمَيْدُ بْنُ عَبْدِ الرَّحْمَنِ، سَمِعَ مُعَاوِيَةَ، يُحَدِّثُ رَهْطًا مِنْ قُرَيْشٍ بِالْمَدِينَةِ، وَذَكَرَ كَعْبَ الأَحْبَارِ فَقَالَ إِنْ كَانَ مِنْ أَصْدَقِ هَؤُلاَءِ الْمُحَدِّثِينَ الَّذِينَ يُحَدِّثُونَ عَنْ أَهْلِ الْكِتَابِ، وَإِنْ كُنَّا مَعَ ذَلِكَ لَنَبْلُو عَلَيْهِ الْكَذِبَ.
sunnah.com