Matthew 17:9-13

9 As they were coming down the mountain, Jesus ordered them, "Tell no one about the vision until after the Son of Man has been raised from the dead."
10 And the disciples asked him, "Why, then, do the scribes say that Elijah must come first?"
11 He replied, "Elijah is indeed coming and will restore all things;
12 but I tell you that Elijah has already come, and they did not recognize him, but they did to him whatever they pleased. So also the Son of Man is about to suffer at their hands."
13 Then the disciples understood that he was speaking to them about John the Baptist.

Matthew 17:9-13 In-Context

7 But Jesus came and touched them, saying, "Get up and do not be afraid."
8 And when they looked up, they saw no one except Jesus himself alone.
9 As they were coming down the mountain, Jesus ordered them, "Tell no one about the vision until after the Son of Man has been raised from the dead."
10 And the disciples asked him, "Why, then, do the scribes say that Elijah must come first?"
11 He replied, "Elijah is indeed coming and will restore all things;
12 but I tell you that Elijah has already come, and they did not recognize him, but they did to him whatever they pleased. So also the Son of Man is about to suffer at their hands."
13 Then the disciples understood that he was speaking to them about John the Baptist.
14 When they came to the crowd, a man came to him, knelt before him,
15 and said, "Lord, have mercy on my son, for he is an epileptic and he suffers terribly; he often falls into the fire and often into the water.