It is narrated from Sayyiduna al-Mustawrid (may Allah be pleased with him); he said: I heard the Messenger of Allah (peace and blessings be upon him), while pointing with his finger, say: "By Allah, this world, compared to the Hereafter, is only like one of you putting his finger into the sea, then taking it out; so let him see what it brings back."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17847