Augustine of Hippo — as excerpted in the Catena Aurea on John 6:15-21

“Mark’s1 account does not contradict this. He says indeed that our Lord told the disciples first to enter the ship, and go before Him over the sea, while He dismissed the crowds, and that when the crowd was dismissed, He went up alone into the mountain to pray: while John places His going up alone in the mountain first, and then says, And when even was now come, His disciples went down unto the sea. But it is easy to see that John relates that as done afterwards by the disciples, which our Lord had ordered before His departure to the mountain.”