Reported by Wabisah bin Ma`bad (RA): I went to Messenger of Allah (peace be upon him) and he asked me, "Have you come to inquire about piety?'' I replied in the affirmative. Then he said, "Ask your heart regarding it. Piety is that which contents the soul and comforts the heart, and sin is that which causes doubts and perturbs the heart, even if people pronounce it lawful and give you verdicts on such matters again and again.''
[Ahmad and Ad-Darmi].