Musnad Ahmad 32:137

Sufyan narrated to us, from al-Zuhri, who heard it from Anas, who said: The Prophet, peace and blessings of Allah be upon him, arrived [in Medina] while I was ten years old, and he died while I was twenty years old. My mothers used to urge me to serve him. He entered upon us, and we milked for him from a domesticated sheep, and it was mixed with water from a well in the house. A Bedouin was to his right, Abu Bakr was to his left, and Umar was to the side. The Messenger of Allah, peace and blessings of Allah be upon him, drank. Then Umar said: "Give to Abu Bakr." But he handed it to the Bedouin and said: "The one on the right, then the one on the right." Sufyan said once: Al-Zuhri said: Anas informed us.