The Sign of Jonah Matthew 12:38-40 38 Then some of the Pharisees and teachers of the law said to him, “Teacher, we want to see a sign from you.” 39 He answered, “A wicked and adulterous generation asks for a sign! But none will be given it except the sign of the prophet Jonah. 40 For as Jonah was three days and three nights in the belly of a huge fish, so the Son of Man will be three days and three nights in the heart of the earth.
Jesus tells them only and evil and adulterous generation seeks after a sign - NONE will be given - EXCEPT what was the sign of the Prophet Jonah. Just as he spent 3 days and 3 nights in the belly of a great fish (whale?) - so will the Son of Man be 3 days and 3 nights in the heart of the earth.
Jesus put a time limit on the days He would be in the grave and He drew a parallel out of Jewish history - not now a mythical story of a man named Jonah swallowed by a great fish but a parallel of His own death and resurrection. He speaks of Jonah as a literal event - not a myth. He basically says, "that's the only sign I'm going to give you - the Son of Man (Jesus) will be 3 days and 3 nights in the heart of the earth (grave), then He will leave the grave - He will rise from the dead.