Musnad Ahmad 595:2

Ibn Numayr narrated to us, Isma'il narrated to us, and Yazid ibn Harun said: Isma'il informed us, from Qays, who said: I heard al-Mustawrid, the brother of Banu Fihr, saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "By Allah, the life of this 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." He meant the one next to the thumb.