Musnad Ahmad 626:65

Sufyan narrated to us, from Ziyad ibn 'Alaqah, who heard Al-Mughirah ibn Shu'bah say: The Messenger of Allah (peace and blessings of Allah be upon him) stood [in prayer] until his feet became swollen. It was said to him: "O Messenger of Allah, Allah has forgiven you your past sins." He replied: "Shall I not be a grateful servant?"