After the voice had spoken, only Jesus was present with them. The disciples kept this to themselves, and in those days they did not tell anyone what they had seen.

Matthew 17:9
As they were coming down the mountain, Jesus commanded them, "Do not tell anyone about this vision until the Son of Man has been raised from the dead."

Mark 9:9
As they were coming down the mountain, Jesus admonished them not to tell anyone what they had seen until the Son of Man had risen from the dead.

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.