Sahih Bukhari Volume 003, Book 040, Hadith Number 554.
Narated By Sahl bin Sad : Once a tumbler (full of milk or water) was brought to Allah's Apostle who drank from it, while on his right side there was sitting a boy who was the youngest of those who were present, and on his left side there were old men. The Prophet asked, "O boy ! Do you allow me to give (the drink) to the elder people (first)?" The boy said, "I will not prefer anybody to have my share from you, O Allah's Apostle!" So, he gave it to the boy.