Musnad Ahmad 32:1154

Yazid ibn Harun and Rawh narrated to us, they said: Hisham ibn Hassan narrated to us, Rawh said: From Ubayd Allah ibn Dihqan, and Yazid said: From Ubayd Allah ibn Dihqan, from Anas ibn Malik, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade that a man should eat with his left hand, or drink with his left hand." Rawh said in his narration: "And that he should drink with his left hand."