Matthew 17:10-13 (Also found in Mark 9:11-13)
Disciples (to Jesus): Why do the scribes say that Elijah must come first?
Jesus: Elijah truly shall come first, and restore all things. But I say to you, That Elijah has come already, and they knew him not, but have done to him whatsoever they wanted. Likewise shall the Son of man suffer of them.
The disciples understood that Jesus spoke to them of John the Baptist.