Musnad Ahmad 595:7

Yahya ibn Sa'id narrated to us, from Isma'il, who said: Qays narrated to me, saying: I heard al-Mustawrid, the brother of Banu Fihr, say: 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 dipping his finger into the sea; so let him see with what it returns."