Musnad Ahmad 595:1

Waki' narrated to us, he said: Isma'il ibn Abi Khalid narrated to us, from Qays, from al-Mustawrid, brother of the Banu Fihr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The world, in comparison to the Hereafter, is only like one of you placing this finger of his into the sea; so let him see with what it returns." And he pointed with his index finger.