Musnad Ahmad 32:181

Yahya ibn Sa'id narrated to us, Malik narrated to us, Ibn Shihab narrated to me, from Anas ibn Malik, that the Prophet, peace and blessings of Allah be upon him, drank while on his right was a Bedouin and on his left was Abu Bakr. He then handed [the drink] to the Bedouin and said, "The right, then the right."