An-Nawwas ibn Sam’an reported: I asked the Messenger of Allah, peace and blessings be upon him, about righteousness and sin. The Prophet said, “Righteousness is good character and sin is what waivers in your heart and you hate for people to find out about it.”

Source: Sahih Muslim 2553

Grade: Sahih (authentic) according to Muslim

عَنِ النَّوَّاسِ بْنِ سَمْعَانَ الأَنْصَارِيِّ قَالَ سَأَلْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنِ الْبِرِّ وَالإِثْمِ فَقَالَ الْبِرُّ حُسْنُ الْخُلُقِ وَالإِثْمُ مَا حَاكَ فِي صَدْرِكَ وَكَرِهْتَ أَنْ يَطَّلِعَ عَلَيْهِ النَّاسُ 

صحيح مسلم كِتَاب الْبِرِّ وَالصِّلَةِ وَالْآدَابِ بَاب تَفْسِيرِ الْبِرِّ وَالْإِثْمِ 2553