Musnad Ahmad 28:417

'Abd al-Razzaq narrated to us, Sufyan informed us, from Mansur, from Hilal ibn Yasaf, from Abu Yahya, from 'Abdullah ibn 'Amr, who said: I came to the Prophet, peace and blessings of Allah be upon him, while he was praying sitting down. I said: "O Messenger of Allah, I have been told that you said: 'The prayer of one who sits is half of the prayer of one who stands,' yet you are praying sitting?" He said: "Yes, but I am not like any one of you."