Abu Umamah (may Allah swt be pleased with him) related: 'A man once asked Allah's Messenger (may Allah's peace and blessings be upon him, his family and companions) 'What is faith?' He replied: 'When your good deeds make you happy & your bad deeds make you sad, then you have faith.' Then the man asked him, 'What is sin?' The Prophet (pbuh) replied: 'When anything troubles your conscience, stop doing it.' (Ahmed)