Sayyiduna Mughira bin Shu’bah (may Allah be pleased with him) narrates: The Prophet Muhammad (peace be upon him) used to offer voluntary prayers at night and would stand for so long that both his feet would become swollen. It was submitted to him: O Messenger of Allah! Do you do this while your past and future (sins) have been forgiven? The Prophet Muhammad (peace be upon him) replied: Should I not be a grateful servant?