Musnad Ahmad 1153:2238

Al-Hasan ibn Musa narrated to us, he said: Abu Hilal narrated to us, from Muhammad ibn Sirin, from Abdullah ibn Shaqiq, from Aisha, the Mother of the Believers, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to pray much while standing and while sitting. So when he prayed standing, he would bow standing, and when he prayed sitting, he would bow sitting."