The answer is: Jonah

Matt 12:38-39 - Referring to the three days and nights that Jonah has spent in the belly of the great fish

When Jesus was confronted by the Scribes and Pharisees demanding a sign, His reference to Jonah is both profound and packed with meaning. Jonah's story isn't just a fantastical adventure; it represents themes of repentance, mercy, and God's unyielding presence in the face of defiance. Jonah's being swallowed by the great fish isn’t merely a quirky Bible story but echoes a deeper narrative about facing consequences while wrapped in divine compassion.

Consider this: Jonah’s whole ordeal led him from a place of rebellion — running from God’s directive to preach to Nineveh — to becoming a prophet of reconciliation. After three days and nights inside the belly of the fish (hinting at the bigger resurrection theme in Jesus's prophecy), Jonah came out ready to do what he had previously resisted. It's no coincidence that Jesus mirrors this cycle — His death and resurrection highlighted a similar path of miraculous deliverance and transformation.

Additionally, the Ninevites’ response to Jonah’s warning, where they immediately repented, makes for an interesting contrast against the demanding religious leaders who seek proof rather than faith. The story serves as a reminder that often, it is those we least expect to respond (like the Ninevites) who exemplify a readiness toward change and belief. This section of Matthew links the Old and New Testament thinkers effortlessly, assuring us that the old stories bear relevance, bursting with connections waiting to be revisited and applied in fresh ways!