Mustadrak al-Hakim — Hadith 8055

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) came to the Prophet (peace and blessings be upon him) while he (peace and blessings be upon him) was lying on a mat whose marks were visible on his (peace and blessings be upon him) blessed side. Sayyiduna Umar (may Allah be pleased with him) said: O Messenger of Allah! If only you would take a softer bed than this. He (peace and blessings be upon him) said: "What have I to do with the world, and what has the world to do with me! By the One in whose Hand is my soul! The likeness of me and the world is only that of a rider travelling on a summer day who stops for a short while under the shade of a tree to rest, then leaves it and moves on."
This hadith is sahih (authentic) on the condition of Imam Bukhari, but the two Shaykhs (Bukhari and Muslim) did not record it. And its supporting narration (shahid) is the hadith of Abdullah bin Mas'ud.