Musannaf Ibn Abi Shayba 27:603

Ibn Numayr narrated to us, he said: Al-A’mash narrated to us, from Habba ibn Juwayn, who said: We were sitting with Ali, and we mentioned some of the sayings of Abdullah [ibn Mas’ud], and the people praised him. They said: "O Commander of the Faithful, we have not seen a man better in character, nor gentler in teaching, nor more intense in piety, nor better in companionship than Ibn Mas’ud." Ali said: "I adjure you by Allah, is this the truth from your hearts?" They said: "Yes." He said: "O Allah, I call You to witness that I say the same as what they said, or even better."