Hadith 27, 40 Hadith an-Nawawi
On the authority of an-Nawas bin Sam’an (may Allah be pleased with him), that the Prophet (peace and blessings of Allah be upon him) said:
And on the authority of Wabisah bin Ma’bad (may Allah be pleased with him) who said:

I came to the Messenger of Allah (peace and blessings of Allah be upon him) and he (peace and blessings of Allah be upon him) said, “You have come to ask about righteousness.” I said, “Yes.” He (peace and blessings of Allah be upon him) said, “Consult your heart. Righteousness is that about which the soul feels at ease and the heart feels tranquil. And wrongdoing is that which wavers in the soul and causes uneasiness in the breast, even though people have repeatedly given their legal opinion [in its favour].”
عَنْ النَّوَّاسِ بْنِ سَمْعَانَ رَضِيَ اللهُ عَنْهُ عَنْ النَّبِيِّ صلى الله عليه و سلم قَالَ: "الْبِرُّ حُسْنُ الْخُلُقِ، وَالْإِثْمُ مَا حَاكَ فِي صَدْرِك، وَكَرِهْت أَنْ يَطَّلِعَ عَلَيْهِ النَّاسُ" رَوَاهُ مُسْلِمٌ [رَوَاهُ مُسْلِمٌ]. وَعَنْ وَابِصَةَ بْنِ مَعْبَدٍ رَضِيَ اللهُ عَنْهُ قَالَ: أَتَيْت رَسُولَ اللَّهِ صلى الله عليه و سلم فَقَالَ: "جِئْتَ تَسْأَلُ عَنْ الْبِرِّ؟ قُلْت: نَعَمْ. فقَالَ: استفت قلبك، الْبِرُّ مَا اطْمَأَنَّتْ إلَيْهِ النَّفْسُ، وَاطْمَأَنَّ إلَيْهِ الْقَلْبُ، وَالْإِثْمُ مَا حَاكَ فِي النَّفْسِ وَتَرَدَّدَ فِي الصَّدْرِ، وَإِنْ أَفْتَاك النَّاسُ وَأَفْتَوْك" . 
حَدِيثٌ حَسَنٌ، رَوَيْنَاهُ في مُسْنَدَي الْإِمَامَيْنِ أَحْمَدَ بْنِ حَنْبَلٍ [رقم:4/227]، وَالدَّارِمِيّ [2/246] بِإِسْنَادٍ حَسَنٍ.