Mark 9:7

And there was a cloud that overshadowed them
Jesus, Moses, and Elias, and also the disciples; who, according to Luke, entered into it, and so were covered by it.

And a voice came out the cloud, saying, this is my beloved Son, hear
him.
This was the voice of God the Father, bearing a testimony to the sonship of Christ; and was directed, not to Moses and Elias, but to the disciples, enjoining them to hear and obey him, who was the end of the law and prophets; was the great prophet Moses had spoken of, and was to be hearkened to, and whom all the prophets had testified of, and in whom they all centred; (See Gill on Matthew 17:5).