Sahih Bukhari Volume 003, Book 040, Hadith Number 542.
Narated By Az-Zuhri : Anas bin Malik said, that once a domestic sheep was milked for Allah's Apostle while he was in the house of Anas bin Malik. The milk was mixed with water drawn from the well in Anas's house. A tumbler of it was presented to Allah's Apostle who drank from it. Then Abu Bakr was sitting on his left side and a bedouin on his right side. When the Prophet removed the tumbler from his mouth, 'Umar was afraid that the Prophet might give it to the bedouin, so he said. "O Allah's Apostle! Give it to Abu Bakr who is sitting by your side." But the Prophet gave it to the bedouin, who was to his right and said, "You should start with the one on your right side."