Rabanus Maurus
Catena Aurea · Matthäusevangelium 17
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 is 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 spake unto them of John the Baptist.
From the mention of His own passion which the Lord had often foretold to them, and from that of His forerunner, which they beheld already accomplished, the disciples perceived that John was set forth to them under the name of Elias; whence it follows; Then understood the disciples that he spake to them of John the Baptist.