Sahih Bukhari Volume 3, Book 40, Hadith Number 541.

Narated By Sahl bin Sad : A tumbler (full of milk or water) was brought to the Prophet 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, will you allow me to give it (i.e. the rest of the drink) to the old men?” The boy said, “O Allah’s Apostle! I will not give preference to anyone over me to drink the rest of it from which you have drunk.” So, the Prophet gave it to him.