From Sahih Bukhari (Hadith 2571):
Narrated Anas:Once Allah's Messenger (ﷺ) 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 Messenger (ﷺ) finished, `Umar said to Allah's Messenger (ﷺ) "Here is Abu Bakr." But Allah's Messenger (ﷺ) 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
Reference: Bukhari 2571