Musnad Ahmad 790:159

Ishaq ibn Yusuf narrated to us, Husayn informed us, from Abdullah ibn Buraydah, from Imran ibn Husayn, that he asked the Messenger of Allah (peace and blessings of Allah be upon him) about the prayer of one who is sitting. He said: "Whoever prays standing, it is better. Whoever prays sitting, he has half the reward of the one who is standing. And whoever prays lying down, he has half the reward of the one who is sitting."