Pray on the ground if you are able to; and if not, then use gestures and make your prostration lower than your bowing

Jābir ibn ‘Abdullāh (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) visited a sick person and saw him pray on a pillow. He took the pillow and threw it away. The man took it, again with the intention to pray on it, and the Messenger of Allah once again threw it away. He then said: "Pray on the ground if you are able to; and if not, then use gestures and make your prostration lower than your bowing."