Sahih al-Bukhari 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.
حَدَّثَنَا خَالِدُ بْنُ مَخْلَدٍ، حَدَّثَنَا سُلَيْمَانُ بْنُ بِلاَلٍ، قَالَ حَدَّثَنِي أَبُو طَوَالَةَ ـ اسْمُهُ عَبْدُ اللَّهِ بْنُ عَبْدِ الرَّحْمَنِ ـ قَالَ سَمِعْتُ أَنَسًا ـ رضى الله عنه ـ يَقُولُ أَتَانَا رَسُولُ اللَّهِ صلى الله عليه وسلم فِي دَارِنَا هَذِهِ، فَاسْتَسْقَى، فَحَلَبْنَا لَهُ شَاةً لَنَا، ثُمَّ شُبْتُهُ مِنْ مَاءِ بِئْرِنَا هَذِهِ، فَأَعْطَيْتُهُ وَأَبُو بَكْرٍ عَنْ يَسَارِهِ، وَعُمَرُ تُجَاهَهُ وَأَعْرَابِيٌّ عَنْ يَمِينِهِ فَلَمَّا فَرَغَ قَالَ عُمَرُ هَذَا أَبُو بَكْرٍ‏.‏ فَأَعْطَى الأَعْرَابِيَّ، ثُمَّ قَالَ ‏
"‏ الأَيْمَنُونَ، الأَيْمَنُونَ، أَلاَ فَيَمِّنُوا ‏"‏‏.‏ قَالَ أَنَسٌ فَهْىَ سُنَّةٌ فَهْىَ سُنَّةٌ‏.‏ ثَلاَثَ مَرَّاتٍ‏.‏