From Matthew 17

Now after six days Jesus took Peter, James, and John
his brother, led them up on a high mountain by themselves;
and He was transfigured before them. His face shone like the
sun, and His clothes became as white as the light.

And behold, Moses and Elijah appeared to them, talking with
Him. Then Peter answered and said to Jesus, “Lord, it is good
for us to be here; if You wish, let us make here three tabernacles:
one for You, one for Moses, and one for Elijah.”

While he was still speaking, behold, a bright cloud overshadowed
them; and suddenly a voice came out of the cloud, saying,
“This is My beloved Son, in whom I am well pleased. Hear Him!”

And when the disciples heard it, they fell on their faces and were
greatly afraid. But Jesus came and touched them and said, “Arise,
and do not be afraid.” When they had lifted up their eyes,
they saw no one but Jesus only.

Now as they came down from the mountain, Jesus commanded them,
saying, “Tell the vision to no one until the Son of Man is risen
from the dead.”

And His disciples asked Him, saying, “Why then do the scribes say that
Elijah must come first?”

Jesus answered and said to them, “Indeed, Elijah is coming first and
will restore all things. But I say to you that Elijah has come already,
and they did not know him but did to him whatever they wished.
Likewise the Son of Man is also about to suffer at their hands.”
Then the disciples understood that He spoke to them of
John the Baptist.