Bede the Venerable — as excerpted in the Catena Aurea on Mark 9:1-8

“(ubi sup.) He then whose preaching, as Moses foretold, every soul that wished to be saved should hear when He came in the flesh, He now come in the flesh is proclaimed by God the Father to the disciples as the one whom they were to hear. There follows, And suddenly, when they had looked round about, they saw no man any more, save Jesus only with themselves; for as soon as the Son was proclaimed, at once the servants disappeared, lest the voice of the Father should seem to have been sent forth to them.”