Musnad Ahmad 962:30

Rawh narrated to us, Hisham bin Abi Abdullah narrated to us, from Yahya bin Abi Kathir, from Zayd bin Salam, from his grandfather Mamtur, from Abu Umamah, that a man asked the Messenger of Allah, may Allah bless him and grant him peace: "What is faith?" He said: "If your good deed pleases you and your evil deed displeases you, then you are a believer." He said: "O Messenger of Allah, what is sin?" He said: "If something pricks your conscience, then leave it."