Theophylact of Ohrid
Catena Aurea · Lukasevangelium 9
And it came to pass about an eight days after these sayings, he took Peter and John and James, and went up into a mountain to pray. And as he prayed, the fashion of his countenance was altered, and his raiment was white and glistering. And, behold, there talked with him two men, which were Moses and Elias: Who appeared in glory, and spake of his decease which he should accomplish at Jerusalem.
Or, He takes these with Him as men who were able to conceal this thing, and reveal it to no one else. But going up into a mountain to pray, He teaches us to pray solitary, and going up, into stooping to earthly things.