Mark 9:6-10

For he did not know how to reply; for they became terrified.

Then a cloud formed, overshadowing them, and a voice came out of the cloud: “This is My beloved Son; listen to Him!”

And suddenly they looked around and saw no one with them anymore, except Jesus alone.

As they were coming down from the mountain, He gave them orders not to relate to anyone what they had seen, until the Son of Man rose from the dead.

They seized upon that statement, discussing with one another what rising from the dead meant.