Musnad Ahmad 32:1568

Hassan narrated to us, Zuhair narrated to us, Abdullah ibn Abd al-Rahman al-Ansari narrated to us, from Anas, who said: "The Messenger of Allah (ﷺ) set out, and with him were Abu Bakr al-Siddiq, Umar, and some people of the Bedouins, until he entered our house. A sheep was milked for him, and some water from our well was poured over it—I think he said: So he drank. Abu Bakr was on his left, Umar was facing him, and on his right was a Bedouin. Umar said: 'O Messenger of Allah, [give to] Abu Bakr.' But he gave it to the Bedouin and said: 'The ones on the right, the ones on the right.'" Anas said to us: "It is a Sunnah, it is a Sunnah."