Musnad Ahmad 626:110

'Abd al-Rahman narrated to us, Sufyan narrated to us, from Ziyad ibn 'Alaqah, who said: I heard al-Mughirah ibn Shu'bah saying: The Prophet, peace and blessings of Allah be upon him, would pray until his feet became swollen. It was said to him: "Has Allah not forgiven you your past and future sins?" He said: "Should I not be a grateful servant?"