Hadith 201
Sayyida Aisha (may Allah be pleased with her) says: The Prophet (peace and blessings be upon him) would stand in prayer at night so much that his blessed feet would swell. So Sayyida Aisha (may Allah be pleased with her) said: O Messenger of Allah! Do you do this even though Allah has already forgiven your past and future sins? The Prophet (peace and blessings be upon him) replied: "Should I not be a grateful servant of Allah?"