Bede the Venerable
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.
Hence He ascends the mountain to pray and be transfigured, to shew that those who expect the fruit of the resurrection, and desire to see the King in His glory, ought to have the dwelling place of their hearts on high, and be ever on their knees in prayer.