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.

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 ﷺ 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.