Wabisah bin Ma`bad (May Allah be pleased with him) reported: I went to Messenger of Allah (PBUH) 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].

Commentary: There are two points about this Hadith. First, it is one of the miracles of Messenger of Allah (PBUH) who discovered the nature of the question before it was put to him. Second, man's heart is the biggest guide to him and he, therefore, should keep his heart illuminated with the light of Faith so as to continue receiving guidance from it.