It is narrated from Sayyiduna Abu Tha'labah al-Khushani (may Allah be pleased with him); he said: I said: O Messenger of Allah, tell me what is lawful for me and what is unlawful for me. He (peace and blessings be upon him) looked me over from head to foot, then said: "Righteousness is that with which the heart is at ease and the soul finds tranquillity; and sin is that with which the heart is not at ease and the soul finds no tranquillity."
Hadith Referenceالمعجم الكبير للطبراني / باب اللام ألف / 18805