Of the Harmony Between Matthew and Mark in the Accounts Given of the Occasion on Which He Spoke to the Disciples Concerning the Coming of Elias
115 Matthew goes on thus: “And His disciples asked Him, saying, Why then say the scribes that Elias must first come? And Jesus answered and said unto them, Elias truly shall first come and restore all things. But I say unto you, that Elias has come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them. Then the disciples understood that He spoke unto them of John the Baptist.” This same passage is given also by Mark, who keeps also the same order; and although he exhibits some diversity of expression, he makes no departure from a truthful representation of the same sense. He has not, however, added the statement, that the disciples understood that the Lord had referred to John the Baptist in saying that Elias had come already.
Source: The Harmony of the Gospels (New Advent)