Musannaf Ibn Abi Shaybah

Hazrat Masruq narrates that Hazrat Abdullah bin Masood (may Allah be pleased with him) came to meet his brother and saw that he was prostrating on a piece of wood. He threw away that piece of wood and said, "Pray by making a gesture as far as your head can reach."

Muhammad says that when a sick person does not have the strength to perform prostration, he should pray by making gestures as far as his head can reach.