Musnad Abu Ya'la al-Mawsili 154:149

Abu Khaythama narrated to us, Jarir narrated to us, from Layth, from Sa'id ibn 'Amir, from 'Abdullah ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was on a journey, and he passed by a pool of water. They drank from it directly with their mouths (kar'u). The Prophet (peace and blessings of Allah be upon him) said: "Do not drink directly with your mouths, but wash your hands and drink, for there is no vessel cleaner than the hand."