Sahih Bukhari Volume 007, Book 069, Hadith Number 516.
Narrated By Anas bin Malik : I saw Allah's Apostle drinking milk. He came to my house and I milked a sheep and then mixed the milk with water from the well for Allah's Apostle. He took the bowl and drank while on his left there was sitting Abu Bakr, and on his right there was a bedouin. He then gave the remaining milk to the bedouin and said, "The right! The right (first)."