Sayyiduna Anas bin Malik (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) came to Madinah I was ten years old, and when he passed away I was twenty years old, and my mother used to urge me to serve him (peace and blessings be upon him). Whenever the Prophet (peace and blessings be upon him) came to our house, we would milk a goat for him (peace and blessings be upon him) and mix it with water from the well in the house. Sayyiduna Abu Bakr (may Allah be pleased with him) was on his (peace and blessings be upon him) left, a Bedouin was on his right, and Sayyiduna Umar (may Allah be pleased with him) was to one side. The Prophet (peace and blessings be upon him) drank, and Sayyiduna Umar (may Allah be pleased with him) said: Give it to Abu Bakr. But he (peace and blessings be upon him) handed it to the Bedouin and said: 'Al-ayman fal-ayman' "The one on the right first, then the one next to him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14667