Musnad Ahmad 594:1

'Abd al-Rahman ibn Mahdi narrated to us, from Mu'awiyah ibn Salih, from Abu 'Abd Allah al-Sulami, who said: I heard Wabisah ibn Ma'bad, a companion of the Prophet, may Allah bless him and grant him peace, say: I came to the Messenger of Allah, may Allah bless him and grant him peace, to ask him about righteousness and sin. He said: "You have come to ask about righteousness and sin." I said: "By the One who sent you with the truth, I did not come to ask you about anything else." He said: "Righteousness is that which the breast expands for, and sin is that which wavers in the breast, even if the people have given you a legal opinion regarding it."