Musnad Ahmad 1153:827

Harun ibn Ma'ruf narrated to us, he said: Ibn Wahb narrated to us, he said: Abu Sakhr narrated to me, from Ibn Qusayt, from Urwah ibn al-Zubayr, from Aisha, she said: When the Messenger of Allah, peace and blessings of Allah be upon him, would pray, he would stand until his feet would swell. Aisha said: "O Messenger of Allah, do you do this when your past and future sins have been forgiven for you?" He said: "O Aisha, shall I not be a thankful servant?"