Anas bin Malik reported,

The Messenger of Allah ﷺ came to us in this house of ours and asked for something to drink. We milked one of our sheep for him, then I mixed it with water from this well of ours and handed it to him. Abu Bakr was on his left, Umar was facing him, and an Arabi was on his right.When he finished drinking, Umar said, "This is Abu Bakr."But he gave what was left to the Arabi, then said, "Those on the right, those on the right. Begin with those on the right."Anas said, "So this is the sunnah, this is the sunnah, this is the sunnah," repeating it three times.

Bukhari 2571

Sahih

حدثنا خالد بن مخلد حدثنا سليمان بن بلال قال حدثني أبو طوالة اسمه عبد الله بن عبد الرحمن قال سمعت

أنسا رضي الله عنه يقول

أتانا رسول الله ﷺ في دارنا هذه فاستسقى فحلبنا له شاة لنا ثم شبته من ماء بئرنا هذه فأعطيته وأبو بكر عن يساره وعمر تجاهه وأعرابي عن يمينه فلما فرغ قال عمر هذا أبو بكر فأعطى الأعرابي فضله ثم قال الأيمنون الأيمنون ألا فيمنوا قال أنس فهي سنة فهي سنة ثلاث مرات

البخاري 2571صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Bukhari
Book
Gifts, their virtue, and encouragement to give them
Chapter
One who asks for water
Numbering
Darussalam 2571
Bukhari 2571