Jerome
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.
It was a tradition of the Pharisees following the Prophet Malachi, that Elias should come before the coming of the Saviour, and bring back the heart of the fathers to the children, and the children to the fathers, and restore all things to their ancient state. The disciples then consider that this transformation which they had seen in the mount was His coming in glory, and therefore it is said, And his disciples asked him, saying, How then say the Scribes that Elias must first come? As though they had said, If you have already come in glory, how is it that your forerunner appears not yet? And this they say chiefly because they see that Elias is departed again.