Mathew 12:38-41
The scribes and Pharisees asked for a sign. Jesus responds that an evil and adulterous generation craves for a sign yet none shall be given to it except the sign of Jonah.
The sign:
Jonah rebelled against God and went in the opposite direction of where the LORD wanted him to go.
Jonah went down into the bowels of the earth for his rebellion. This was akin to death.
He was raised from the dead and went on to preach a message of repentance.
An entire city (Ninevah) changed and became clean in the eyes of God and were saved.
So too, Jesus was killed (not for rebellion to God, but rebellion to man).
He was put into the bowels of the earth for three days.
He came back and continued His message of repentance.
Many hear and change and are made right in the eyes of God.
Jonah is a type and shadow of Jesus. This is the sign of Jonah.