Musnad Ahmad 1153:2255

Shuja' ibn al-Walid narrated to us, from Layth ibn Abi Sulaym, from 'Abd al-Rahman ibn al-Qasim, from al-Qasim, from 'Aishah, who said: "When the Prophet of Allah, peace and blessings of Allah be upon him, began the prayer standing, he prayed standing, and when he began the prayer sitting, he prayed sitting."