“This is he of whom it is written, Behold, I send my messenger before your face, Who will prepare your way before you.”
For he was the one of whom God had said that he was His messenger, sent before His very eyes, to prepare the way for God to act and to enable Israel to behold its God (Malachi 3:1; compare Isaiah 40:3-5 as in Luke 3:4-6).
The actual quotation is a combination of the Hebrew text for Malachi 3:1 slightly altered and with a slight addition from Exodus 23:20. The messenger will come from God, and like God he will go forward to prepare their way. The same combination is found in Mark 1:2; Matthew 11:10. Possibly it was as contained in a list of prophecies or proof texts compiled by the early church or produced by the Apostles.