Sahih Muslim 36:165

Anas b. Malik reported:

Allah's Messenger (ﷺ) came to our house and he asked for a drink. We milked a goat for him and then mixed it (the milk) with the water of this well of mine. I gave it to Allah's Messenger (ﷺ) and he drank it, while Abu Bakr was on his left and 'Umar was in front of him, and a desert Arab was on his right. When Allah's Messenger (ﷺ) had finished the drink, Umar said: Allah's Messenger, here is Abu Bakr, give him to drink; but Allah's Messenger (ﷺ) gave it to the desert Arab and he left out Abu Bakr and Umar. And Allah's Messenger (ﷺ) said: Those on the right, those on the right, those on the right (deserve preference). Anas said: This is the Sunnah, this is the Sunnah, this is the Sunnah.