Musnad Ahmad 26:162

Yazid narrated to us, Mas'udi informed us, from 'Amr ibn Murrah, from Ibrahim al-Nakha'i, from 'Alqamah, from 'Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) lay down on a reed mat, and it left a mark on his side. When he woke up, I began to wipe his side and said: "O Messenger of Allah, would you not have informed us so that we could have spread something for you on the mat?" The Messenger of Allah (peace and blessings of Allah be upon him) said: "What do I have to do with the world? What am I and the world? My likeness and the likeness of the world is only that of a rider who sought shade under a tree, then departed and left it."