Musnad Ahmad 28:332

Waki’ narrated to us, from Sufyan, from Habib ibn Abi Thabit, from a sheikh whose kunya was Abu Musa, from Abdullah ibn Amr—Sufyan said: I think he meant—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."