Musannaf Ibn Abi Shayba 3:2256

Waki' narrated to us, from Sufyan, from Habib ibn Abi Thabit, from a sheikh whose kunya was Abu Musa, from Abdullah ibn Amr, who said: I think he attributed it to the Prophet (may Allah bless him and grant him peace), who said: "The prayer of the one sitting is half of the prayer of the one standing."