Musnad Ahmad 1153:316

Asbat narrated to us, he said: Sufyan narrated to us, from Ibrahim ibn Muhajir, from the guide of al-Sa’ib ibn ‘Abdullah, from al-Sa’ib, he said: I entered upon ‘Aishah, and she narrated to us that 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."