A man asked the Prophet ﷺ, "What is sin?"He replied, "Whenever something stirs unease in your heart, leave it."The man asked, "What is faith?"He answered, "If your bad deed grieves you and your good deed pleases you, then you are a believer."
سأل رجل النبي ﷺ فقال: ما الإثم؟ فقال: إذا حك في نفسك شيء فدعه. قال: فما الإيمان؟ قال: إذا ساءتك سيئتك، وسرتك حسنتك، فأنت مؤمن.
Imam Ahmad (22159) and Tabarani in Al-Kabir (7539) narrated it, and Ibn Hibban graded it sahih (176), all of them through routes going back to Yahya bin Abu Kathir, from Zayd bin Salam, from his grandfather Mamtur, who said, "I heard Abu Umama" (and he related the hadith). Hakim also narrated it (1/14) through other routes and said, "All these hadiths are sahih and connected, meeting the standard of the two Shaykhs."