What Matthew has done here at 11:10 is combine Malachi 3:1 and Exodus 23:20. The first half of Matthew's quotation is identical to the Greek Septuagint of Exodus 23:20. The second half of Matthew's quotation is not identical to the Greek Septuagint of Malachi 3:1.
MATTHEW 11:10: John the Baptist, the Messenger Who Prepared the Way for the Messiah-King
