Matthew 17:1-13 (ESV)
17:1 And after six days Jesus took with him Peter and James, and John his brother, and led them up a high mountain by themselves.2 And he was transfigured before them, and his face shone like the sun, and his clothes became white as light. 3 And behold, there appeared to them Moses and Elijah, talking with him. 4 And Peter said to Jesus, “Lord, it is good that we are here. If you wish, I will make three tents here, one for you and one for Moses and one for Elijah.” 5 He was still speaking when, behold, a bright cloud overshadowed them, and a voice from the cloud said, “This is my beloved Son,  with whom I am well pleased; listen to him.” 6 When the disciples heard this, they fell on their faces and were terrified. 7 But Jesus came and touched them, saying, “Rise, and have no fear.” 8 And when they lifted up their eyes, they saw no one but Jesus only.
9 And as they were coming down the mountain, Jesus commanded them, “Tell no one the vision, until the Son of Man is raised from the dead.” 10 And the disciples asked him, “Then why do the scribes say that first Elijah must come?” 11 He answered, “Elijah does come, and he will restore all things. 12 But I tell you that Elijah has already come, and they did not recognize him, but did to him whatever they pleased. So also the Son of Man will certainly suffer at their hands.” 13 Then the disciples understood that he was speaking to them of John the Baptist.