http://www.crossmarks.com/brian/luke9x28.htm
Exodus 34-29-35; Psalm 99; ... fail to understand Jesus' message about his passion (v. 45), spar over relative status (v. 46), and attempt to exercise control over one whose ministry succeeds where theirs had not (v. 49). ... Only Luke has Jesus going up the mountain "to pray" and it is while he is "praying" that he is transfigured