Musnad Ahmad 100:2

'Abd al-Rahman ibn Mahdi narrated to us, Sufyan narrated to us, from Ibrahim—meaning Ibn Muhajir—from Mujahid, from the leader of al-Sa'ib, from al-Sa'ib, from the Prophet, peace and blessings of Allah be upon him, who said: "The prayer of the one sitting is half of the prayer of the one standing."