‘Ā'ishah and Al-Mughīrah ibn Shu‘bah (may Allah be pleased with both of them) reported: The Prophet (may Allah's peace and blessings be upon him) used to stand in prayer at night until the skin of his feet would crack. I (‘Ā'ishah) asked him: "O Messenger of Allah, why are you doing this while your past and future sins have been forgiven for you?" He said: "Should I not like to be a grateful slave (of Allah)?"
The Prophet (may Allah's peace and blessings be upon him) used to stand and pray at night until the skin on his feet would crack. ‘Aishah (may Allah be pleased with her) said to him - thinking that he was worshiping Allah out of fear of sins and seeking forgiveness and mercy from Allah, while Allah has already forgiven his sins and he is in no need of that - "Why are you doing this, O Messenger of Allah, while Allah has forgiven your past and future sins?" He replied: "Should I not be a thankful slave." So this worship was an expression of his gratitude towards Allah for His forgiveness.