As the crowds increased, Jesus said, ‘This is a wicked generation. It asks for a sign, but none will be given it except the sign of Jonah. For as Jonah was a sign to the Ninevites, so also will the Son of Man be to this generation. The Queen of the South will rise at the judgment with the people of this generation and condemn them, for she came from the ends of the earth to listen to Solomon’s wisdom; and now something greater than Solomon is here. The men of Nineveh will stand up at the judgment with this generation and condemn it, for they repented at the preaching of Jonah; and now something greater than Jonah is here. – Luke 11:29-32
The sign of Jonah refers to the connection between the three days Jonah was was in the belly of the great fish and the three days Jesus would be in the tomb before He rose from the dead.