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.
This is an example of a composite reference, combining Exodus 23:20 ("Behold, I send an Angel before thee, to keep thee in the way, and to bring thee into the place which I have prepared"), Malachi 3:1 ("Behold, I will send my messenger, and he shall prepare the way before me: and the LORD, whom ye seek, shall suddenly come to his temple, even the messenger of the covenant, whom ye delight in: behold, he shall come, saith the LORD of hosts") and Isaiah 40:3 ("The voice of him that crieth in the wilderness, Prepare ye the way of the LORD, make straight in the desert a highway for our God").
This brace of quotations is discussed by Bart Ehrman in his blog here because of the interesting textual history of the gospel. It appears that Mark only mentioned Isaiah, and scribes subsequently "corrected" this to "the prophets".
