Sayyiduna Nawwas bin Sam'an al-Ansari (may Allah be pleased with him) said: I asked the Messenger of Allah (peace and blessings be upon him) about righteousness and sin, and he (peace and blessings be upon him) said: "Righteousness is good character, and sin is what wavers in your heart and you dislike that people should come to know of it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20785