Musnad Abu Ya'la al-Mawsili 154:71

Wasil ibn 'Abd al-A'la narrated to us, Ibn Fudayl narrated to us, from Layth, from Sa'id ibn 'Amir, from Ibn 'Umar, who said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "Indeed, there are more than seventy callers in my Ummah, all of whom are callers to the Fire. If I wished, I would inform you of their fathers and their tribes." He said: Then we passed by a pond, and we began to drink from it directly with our mouths (kar'). The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not drink directly with your mouths, but wash your hands, then drink from it, for there is no vessel purer than the hand."