Musnad Ahmad 32:1293

'Abd al-Malik ibn 'Amr narrated to us, 'Abdullah ibn Ja'far narrated to us, from Isma'il ibn Muhammad, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) went out to some people while they were praying sitting down due to illness. He said: "The prayer of the one who sits is half of the prayer of the one who stands."