Musnad Ahmad 790:168

'Abd al-Samad narrated to us, my father narrated to us, Husayn narrated to us, from Ibn Buraydah, and 'Affan said: 'Abd al-Warith narrated to us, Husayn al-Mu'allim narrated to us, 'Abdullah ibn Buraydah narrated to me, he said: 'Imran ibn Husayn narrated to me—and he was a man who suffered from hemorrhoids—he said: I asked the Messenger of Allah, peace and blessings of Allah be upon him, about the prayer of a man while he 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."