Matthew 17:5

While he was yet speaking, behold, a bright cloud overshadowed them: and behold, a voice out of the cloud, saying, This is my beloved Son, in whom I am well pleased; hear ye him.

The triple "behold" is significant. Miracle was piled upon miracle in the succession of astounding occurrences. Here is a manifestation of the Trinity almost as definite as that at the baptismal scene in Matthew 3:16. Christ was present, radiant in heavenly light; the Father spake out of heaven; and the cloud strongly suggests the Holy Spirit, although it is not so identified. The frightened apostles fell on their faces in abject terror at that overwhelming display of divine power. The exact nature of the bright cloud is not known, but Peter called it "the excellent glory" (2 Peter 1:17).

The words out of the cloud were the same as those at Jesus' baptism, except that the words "Hear ye him" were added. Like all of God's commandments, this is exclusive and means "Do not hear Moses; do not hear Elijah, etc."