Musnad Ahmad 1153:1832

Hajjaj narrated to us, he said: Shareek informed us, from Ibrahim ibn Muhajir, from Mujahid, from the freed slave woman of As-Sa’ib, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The prayer of the one sitting is half of the prayer of the one standing."