“After saying farewell,
He went up
On the mountain
καὶ ἀποταξάμενος αὐτοῖς ἀπῆλθεν εἰς τὸ ὄρος προσεύξασθαι.
This incident about Jesus praying alone can also be found in Matthew, chapter 14:23. After he had bid farewell to his disciples and the crowd (καὶ ἀποταξάμενος αὐτοῖς), Jesus went up the mountain (ἀπῆλθεν εἰς τὸ ὄρος) to pray (προσεύξασθαι). Jesus wanted to be alone with his Father to pray. It is not clear where this mountain was.