Luke 9:36

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.

Cross-reference

Matthew 17:9 records Jesus' command to keep silent about the vision, explaining why the disciples told no one in Luke 9:36.

Mark 9:10 echoes Luke 9:36, showing the disciples kept the transfiguration to themselves and pondered the resurrection.