Theophylact of Ohrid
Catena Aurea · Lukasevangelium 9
But Peter and they that were with him were heavy with sleep: and when they were awake, they saw his glory, and the two men that stood with him. And it came to pass, as they departed from him, Peter said unto Jesus, Master, it is good for us to be here: and let us make three tabernacles; one for thee, and one for Moses, and one for Elias: not knowing what he said. While he thus spake, there came a cloud, and overshadowed them: and they feared as they entered into the cloud. And there came a voice out of the cloud, saying, This is my beloved Son: hear him. And when the voice was past, Jesus was found alone. And they kept it close, and told no man in those days any of those things which they had seen.
While Christ is engaged in prayer, Peter is heavy with sleep, for he was weak, and did what was natural to man; as it is said, But Peter and they that were with him were heavy with sleep. But when they awake, they behold His glory, and the two men with Him; as it follows, And when they were awake, they saw his glory, and the two men, that stood with him.