Musnad Ahmad 25:885

'Abd al-Samad, Abu Sa'id, and 'Affan narrated to us, they said: Thabit narrated to us, Hilal narrated to us, from 'Ikrimah, from Ibn 'Abbas, that the Messenger of Allah (may Allah bless him and grant him peace) was visited by 'Umar while he was on a mat that had left an impression on his side. He said: "O Prophet of Allah, if only you had taken a mattress softer than this?" He said: "What do I have to do with the world? My likeness and the likeness of the world is only that of a rider traveling on a hot summer day, who sought shade under a tree for an hour of the day, then departed and left it."