From: Luke 11:29-32
The Sign of Jonah
 When the crowds were increasing, He (Jesus) began to say, "This generation is an evil generation; it seeks a sign, but no sign shall be given to it except the sign of Jonah.  For as Jonah became a sign to the men of Nineveh, so will the Son of Man be to this generation.  The queen of the South will arise at the judgment with the men of this generation and condemn them; for she came from the ends of the earth to hear the wisdom of Solomon, and behold, something greater than Solomon is here.  The men of Nineveh will arise at the judgment with this generation and condemn it; for they repented at the preaching of Jonah, and behold, something greater than Jonah is here."
29-32. Jonah was the prophet who led the Ninevites to do penance: his actions and preaching they saw as signifying that God had sent him (cf. note on Matthew 12:41-42).
[Note on Matthew 12:41-42 states:
41-42. Nineveh was a city in Mesopotamia (modern Iraq) to which the prophet Jonah was sent. The Ninevites did penance (John 3:6-9) because they recognized the prophet and accepted his message; whereas Jerusalem does not wish to recognize Jesus, of whom Jonah was merely a figure. The queen of the South was the queen of Sheba in southwestern Arabia, who visited Solomon (1 Kings 10:1-10) and was in awe of the wisdom with which God had endowed the King of Israel. Jesus is also prefigured in Solomon, whom Jewish tradition saw as the epitome of the wise man. Jesus' reproach is accentuated by the example of pagan converts, and gives us a glimpse of the universal scope of Christianity, which will take root among the Gentiles.
There is a certain irony in what Jesus says about "something greater" than Jonah or Solomon having come: really, He is infinitely greater, but Jesus prefers to tone down the difference between Himself and any figure, no matter how important, in the Old Testament.]
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.