Sayyiduna Abu Umamah (may Allah be pleased with him) said: A man said: O Messenger of Allah, what is sin? He (peace and blessings be upon him) said: "Whatever troubles your heart, leave it."
He said: Then what is faith? He (peace and blessings be upon him) said: "Whoever is grieved by his evil deed and made happy by his good deed is a believer."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7508