Musannaf Ibn Abi Shayba 33:1327

Sa'id ibn 'Abd Allah narrated to us, from Nusayr, from Bakr, who said: Al-Rabi' used to say: "O Bakr ibn Ma'iz, O Bakr, restrain your tongue except for what is for you and not against you. For I have become suspicious of people regarding my religion. Obey Allah in what you know, and as for what has been kept exclusively to Himself away from you, entrust it to the One who knows it. For I am more fearful for you regarding intentional acts than I am for you regarding mistakes. The best of you today is not good because of his own goodness, but rather he is better than someone else who is worse than him. You have not grasped everything that Allah revealed to Muhammad, may Allah bless him and grant him peace, nor do you know what everything you read is. The secrets that are hidden from the people, and they are in a valley [known only] to Allah—seek their remedy." Then he would say to himself: "And what is their remedy? It is that you repent to Allah, then do not return [to the sin]."