Anas reported, "The Messenger of Allah ﷺ came to us at our home and asked for something to drink. We milked a sheep for him, and I mixed the milk with water from this well of ours. I then handed the cup to the Messenger of Allah ﷺ, and he drank.Abu Bakr was on his left, Umar was facing him, and an Arabi was on his right. When the Messenger of Allah ﷺ finished drinking, Umar said, 'Messenger of Allah, here is Abu Bakr,' indicating that he should be given the cup next. But the Messenger of Allah ﷺ gave it to the Arabi, passing over Abu Bakr and Umar, and he said, 'Those on the right, those on the right, those on the right.'"Anas said, "So it is the sunnah, so it is the sunnah, so it is the sunnah."
أتانا رسول الله ﷺ في دارنا، فاستسقى، فحلبنا له شاء، ثم شبته من ماء بثري هذه، قال: فأعطيت رسول الله ﷺ فشرب رسول الله ﷺ، وأبو بكر عن يساره، وعمر وجاهه، وأعرابي عن يمينه، فلما فرغ رسول الله ﷺ من شربه، قال عمر: هذاأبو بكر يا رسول الله يريه إياه، فأعطى رسول الله ﷺ الأعرابي، وترك أبا بكر وعمر، وقال رسول الله ﷺ: الأيمنون، الأيمنون، الأيمنون. قال أنس: فهي سنة، فهي سنة، فهي سنة.
Muslim narrated it in the Book of Drinks (2029:126) through several routes from Abdullah bin Abdul-Rahman bin Mamar bin Hazm, Abu Tuwala the Ansari, that he heard Anas bin Malik narrating, and he mentioned it.His statement, "that is the sunnah," means that precedence should begin with the one who is greatest in age, knowledge, or leadership, then proceed to whoever is on his right. This is the sunnah.