From Sahih Bukhari (Hadith 5619):
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)
Reference: Bukhari 5619