Musnad Abu Ya'la al-Mawsili 118:65

Ishaq narrated to us, Sufyan narrated to us, he said: Al-Zuhri heard Anas saying: The Messenger of Allah (may Allah bless him and grant him peace) 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 our house, so we milked for him from a domestic sheep, and we mixed its milk for him with water from the well of the house. Abu Bakr was on his right, and Umar was to the side. The Prophet (may Allah bless him and grant him peace) drank, then Umar said: "O Messenger of Allah, give it to Abu Bakr." But he handed it to a Bedouin and said: "The one on the right, then the one on the right."