Musnad Ahmad 27:1283

Ibrahim ibn Abi al-Abbas narrated to us, Sharik narrated to us, from Abu Ishaq, from al-Bahiy, Sharik said: I believe it is from Abdullah ibn Umar, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to pray on the khumrah (a small prayer mat made of palm leaves)."