Musnad Ahmad 962:63

Isma'il narrated to us, Hisham al-Dastawa'i informed us, from Yahya ibn Abi Kathir, from Zayd ibn Salam, from his grandfather Mamtur, from Abu Umamah, who said: A man said: "O Messenger of Allah, 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 chest, then leave it."