Musnad Ahmad 1007:18

Isma'il narrated to us, al-Dastawa'i narrated to us, from Yahya ibn Abi Kathir, from 'Abdullah ibn Abi Qatadah, from Abu Qatadah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you drinks, let him not breathe into the vessel. And when he goes to the privy, let him not touch his private part with his right hand. And when he cleans himself (after relieving himself), let him not clean himself with his right hand."