Musnad Abu Ya'la al-Mawsili 118:17

Abu Khaythama narrated to us, Sufyan ibn 'Uyaynah narrated to us, from al-Zuhri, from Anas, who said: The Prophet (peace 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 our house, and we milked for him from a domestic sheep, and it was mixed with water from a well in the house. The Messenger of Allah (peace be upon him) drank, and Umar said to him—while Abu Bakr was to his left—"O Messenger of Allah, give to Abu Bakr." But he gave it to a Bedouin who was to his right, and the Messenger of Allah (peace be upon him) said: "The right, then the right."