Musannaf Ibn Abi Shayba 3:440

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from Husayn, who said: I asked ‘Amir about the prayer of the sick person. He said: If he is not able to place his forehead upon the ground, then let him gesture with a gesture, and let him make the prostration lower than the bowing.