As it is written in the prophets - Mark mentions “prophets” here without specifying which. The places are found in Malachi 3:1, and in Isaiah 41:3. See the notes at Matthew 3:3.
Related Commentaries of Mark 1:2
Mark 1:2-3
2 As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee.
3 The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.