Musnad Ahmad 28:36

Yahya narrated to us, from Sufyan, Mansur narrated to us, from Hilal ibn Yasaf, from Abu Yahya, from Abdullah ibn Amr: I saw the Messenger of Allah (peace and blessings of Allah be upon him) praying while sitting. I said to him, "It has been reported to me that you say: 'The prayer of one who sits is half the prayer of one who stands'?" He said, "I am not like you."