Bukhari Collection: ::Book Number 3:: Hadith # 745
Narrated Anas:Once Allah's Apostle visited us in this house of ours and asked for something to drink. We milked one of our sheep and mixed it with water from this well of ours and gave it to him. Abu Bakr was sitting on his left side and `Umar in front of him and a bedouin on his right side. When Allah's Apostle finished, `Umar said to Allah's Apostle "Here is Abu Bakr." But Allah's Apostle gave the remaining milk to the bedouin and said twice, "The (persons on the) right side! So, start from the right side." Anas added, "It is a Sunna (the Prophet's traditions)" and repeated it thrice.