Musnad Ahmad 1007:131

Yahya ibn Sa'id narrated to us, Hisham narrated to us, Yahya narrated to me, from 'Abdullah ibn Abi Qatadah, that Abu Qatadah informed him, from the Prophet, may Allah bless him and grant him peace, who said: "When one of you drinks, let him not breathe into the vessel. And when one of you comes to the privy, let him not cleanse himself with his right hand." And Abu 'Amir said: "And let none of you touch his private part with his right hand."