Matthew 11:10

This is he, of whom it is written, Behold, I send my messenger before thy face, Who shall prepare the way before thee.

Christ's selection of this prophecy from Malachi 3:1 and application of it to John proves two things: (1) that John the Baptist is that first messenger mentioned in that passage, and (2) that Jesus Christ is the Lord, "the messenger of the covenant" who even then had suddenly come to his temple.