Sahih Muslim Book 23, Hadith Number 5034.

Chapter : It is desirable to circulate water or milk (in an assembly) from the right-hand side of the one who serves.

Anas b. Malik reported: Allah’s Messenger (may peace be upon him) 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 (may peace be upon him) 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 (may peace be upon him) had finished the drink, Umar said: Allah’s Messenger, here is Abu Bakr, give him to drink; but Allah’s Messenger (may peace be upon him) gave it to the desert Arab and he left out Abu Bakr and Umar. And Allah’s Messenger (may peace be upon him) 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.