Sahih Muslim Book 23, Hadith Number 5035.

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

Sahl b. Sa’d Sa’idi reported that Allah’s Messenger (may peace be upon him) was given a drink, and he drank from that, and there was on his right side a boy and on his left some old men. He said to the boy: Do you permit me to give it to them (the old men), but that boy said: by God. I will not give preference at your hand over me in my share. He (the narrator) said that Allah’s Messenger (may peace be upon him) then gave it in his hand.