Sahih Bukhari Volume 3, Book 42, Hadith 2353

حَدَّثَنَا أَبُو الْيَمَانِ، أَخْبَرَنَا شُعَيْبٌ، عَنِ الزُّهْرِيِّ، قَالَ حَدَّثَنِي أَنَسُ بْنُ مَالِكٍ ـ رضى الله عنه ـ أَنَّهَا حُلِبَتْ لِرَسُولِ اللَّهِ صلى الله عليه وسلم شَاةٌ دَاجِنٌ وَهْىَ فِي دَارِ أَنَسِ بْنِ مَالِكٍ، وَشِيبَ لَبَنُهَا بِمَاءٍ مِنَ الْبِئْرِ الَّتِي فِي دَارِ أَنَسٍ، فَأَعْطَى رَسُولَ اللَّهِ صلى الله عليه وسلم الْقَدَحَ فَشَرِبَ مِنْهُ، حَتَّى إِذَا نَزَعَ الْقَدَحَ مِنْ فِيهِ، وَعَلَى يَسَارِهِ أَبُو بَكْرٍ وَعَنْ يَمِينِهِ أَعْرَابِيٌّ فَقَالَ عُمَرُ وَخَافَ أَنْ يُعْطِيَهُ الأَعْرَابِيَّ أَعْطِ أَبَا بَكْرٍ يَا رَسُولَ اللَّهِ عِنْدَكَ‏.‏ فَأَعْطَاهُ الأَعْرَابِيَّ الَّذِي عَلَى يَمِينِهِ، ثُمَّ قَالَ ‏"‏ الأَيْمَنَ فَالأَيْمَنَ ‏"‏‏.‏

Narrated 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."