John Gill

And when the disciples heard it,.... The voice out of the cloud, and which they apprehended came from God, and was uttered with so much majesty: they fell on their face: not so much out of reverence, or for the sake of adoration, but as persons struck with astonishment and fear, and were as half dead; and so fell with their faces fiat to the ground, not being able to stand before God, to behold his majesty, and hear his voice: and were sore afraid: they were filled with fear, when, awaking out of their sleep, they saw the surprising glory of Christ, and of the two men that were with him, insomuch that they knew not what to think, or say; and so they were when they entered into the cloud, and still more upon hearing the voice of God himself, even though it was a voice of love, grace, and mercy; see Deu 5:24. But yet they were not struck with so much amazement and surprise, as not to know what was said; for they distinctly heard the words, rightly understood, and faithfully related them; from whom the evangelists had them, and which Peter perfectly remembered, and recorded many years after.