Musnad Ahmad 962:23

Ibrahim ibn Khalid narrated to us, Rabah narrated to us, from Ma'mar, from Yahya ibn Abi Kathir, from Zayd ibn Salam, from his grandfather, who said: I heard Abu Umamah say: A man asked the Prophet, may God bless him and grant him peace, and said: "What is sin?" He said: "If something scratches [bothers] your soul, then leave it." He said: "Then what is faith?" He said: "If your evil deed distresses you and your good deed pleases you, then you are a believer."