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, "When your bad deed grieves you and your good deed pleases you, then you are a believer."
حدثنا إبراهيم بن خالد حدثنا رباح عن معمر عن يحيى بن أبي كثير عن زيد بن سلام عن جده قال سمعت
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
Its isnad is sahih. Rabah is Ibn Zayd Sanani, the Qurashi, their freed client, a trustworthy and excellent narrator whom they praised highly.The hadith was narrated by Tirmidhi (4/404, no. 2165) and Tabarani in Al-Kabir (8/138, no. 7539), and Ibn Hibban (103, Mawarid), and Hakim graded it sahih (1/14 and 4/99). Haythami (1/86) did not attribute it to Ahmad, but attributed it to Tabarani instead, and said, "Yahya bin Abu Kathir practices tadlis, even though he is among the narrators of the Sahih."