The one on the right should drink first
حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنِي مَالِكٌ، عَنِ ابْنِ شِهَابٍ، عَنْ أَنَسِ بْنِ مَالِكٍ ـ رضى الله عنه ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم أُتِيَ بِلَبَنٍ قَدْ شِيبَ بِمَاءٍ، وَعَنْ يَمِينِهِ أَعْرَابِيٌّ وَعَنْ شِمَالِهِ أَبُو بَكْرٍ، فَشَرِبَ، ثُمَّ أَعْطَى الأَعْرَابِيَّ، وَقَالَ " الأَيْمَنَ الأَيْمَنَ ".
Narrated Anas bin Malik: Milk mixed with water was brought to Allah's Messenger (ﷺ) while a bedouin was on his right and Abu Bakr was on his left. He drank (of it) and then gave (it) to the bedouin and said, 'The right" "The right (first).