It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) visited a sick man and saw that he was praying on a pillow. He (peace and blessings be upon him) took hold of the pillow and threw it away. The rest of the hadith is the same, except that in it he (peace and blessings be upon him) said: "Pray on the ground if you are able."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3670