Musannaf Ibn Abi Shayba 3:2259

Yahya ibn Adam narrated to us, from Zuhayr, from Ibrahim ibn Muhajir, from Mujahid, that al-Sa’ib asked ‘Aishah about the prayer of the one sitting. She said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The prayer of the one sitting is half of the prayer of the one standing."