Musannaf Ibn Abi Shayba 33:1331

Sa'id ibn 'Abd Allah narrated to us, from Nusayr, from Bakr, who said: Ibn al-Kawwa' said to Rabi' ibn Khuthaym, "We do not see you disparaging anyone, nor do you find fault with them." He said, "Woe to you, O Ibn al-Kawwa'! I am not satisfied with my own self, so how could I be free from disparaging myself to disparaging the people? Indeed, the people feared Allah regarding the sins of the servants, yet they felt secure regarding their own sins."