Then the disciples understood that He was speaking to them about John the Baptist.

Matthew 3:1
In those days John the Baptist came, preaching in the wilderness of Judea

Matthew 17:12
But I tell you that Elijah has already come, and they did not recognize him, but have done to him whatever they wished. In the same way, the Son of Man will suffer at their hands."

Matthew 17:14
When they came to the crowd, a man came up to Jesus and knelt before Him.

Then the disciples understood that he spoke to them of John the Baptist.