Sahih Muslim Book 23, Hadith Number 5032.

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 that there was brought to Allah’s Messenger (may peace be upon him) a cup of milk mixed with water, while there was on his right a desert Arab and on his left Abu Bakr. He (the Holy Prophet) drank; he then gave it to the desert Arab and said: (Give to one) who is on the right, then again who is on the right.