Matthew 17:13

Origen Commentary on Matthew Book XIII

And these things indeed as about Elijah the disciples asked and the Saviour answered, but when they heard they understood that the words, "Elijah has already come," and that following which was spoken by the Saviour, had reference to John the Baptist.[4]

Origen Commentary on Matthew Book XIII

may be referred to the spirit of Elijah that was in John; as also the three disciples who had gone up with Him understood that He spake to them about John the Baptist.[28]