Arabic text
عَنِ النَّوَّاسِ بْنِ سِمْعَانَ الْأَنْصَارِيِّ رضي الله عنه قَالَ: سَأَلْتُ رَسُولَ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ عَنِ الْبِرِّ وَالْإِثْمِ، فَقَالَ: «الْبِرُّ حُسْنُ الْخُلُقِ، وَالْإِثْمُ مَا حَاكَ فِي صَدْرِكَ، وَكَرِهْتَ أَنْ يَطَّلِعَ عَلَيْهِ النَّاسُ».
Translation
An-Nawwās ibn Sim‘ān al-Ansāri (may Allah be pleased with him) reported: I asked the Messenger of Allah (may Allah's peace and blessings be upon him) about righteousness and sinfulness, and he said, "Righteousness is good morals, and sinfulness is what your heart is not at ease with, and you hate that people know about it."
Explanation
The Prophet (may Allah's peace and blessings be upon him) was asked about righteousness and sinfulness so he said: The greatest aspect of righteousness is adopting good morals with Allah through piety and with people through the endurance of harm, avoidance of anger, having a cheerful face, speaking good words, maintaining ties, obedience, kindness, charity, kind treatment, and good companionship. As for sinfulness, it refers to suspicious matters that move and waver within the soul, the breast does not feel at ease with, the heart feels doubtful about and fearful of its being a sin, and one is not willing to show it to the elite, or the best and the most perfect among people given its ugliness. This is because the soul has a natural inclination to show people its good aspects, so when it hates making some of its acts known to people, then this is a sin that is devoid of goodness.
Source: HadeethEnc.com