Sahih Muslim Book 23, Hadith Number 5033.

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

Anas reported: The Apostle of Allah (may peace he upon him) came to Medina when I was ten years old and he died when I was twenty years old. My mother exhorted me to serve him. He (the Holy Prophet) came to our house, and we ruined a flabby goat for him and mixed it (the milk) with water from the well of the house. Allah’s Messenger (may peace be upon him) drank that. Umar and Abu Bakr on his left side said to him: Allah’s Messenger, give it to Abu Bakr, but he (the Holy Prophet) gave it to the desert Arab who was on his right. Allah’s Messenger (may peace be upon him) said: He who is on the right, then he who is on the right.