Matthew 16:1-12

16 And the Pharisees and Sadducees came, and to test him they asked him to show them a sign from heaven. 2 He answered them, “When it is evening, you say, ‘It will be fair weather, for the sky is red.’ 3 And in the morning, ‘It will be stormy today, for the sky is red and threatening.’ You know how to interpret the appearance of the sky, but you cannot interpret the signs of the times. 4 An evil and adulterous generation seeks for a sign, but no sign will be given to it except the sign of Jonah.” So he left them and departed.
5 When the disciples reached the other side, they had forgotten to bring any bread. 6 Jesus said to them, “Watch and beware of the leaven of the Pharisees and Sadducees.” 7 And they began discussing it among themselves, saying, “We brought no bread.” 8 But Jesus, aware of this, said, “O you of little faith, why are you discussing among yourselves the fact that you have no bread? 9 Do you not yet perceive? Do you not remember the five loaves for the five thousand, and how many baskets you gathered? 10 Or the seven loaves for the four thousand, and how many baskets you gathered? 11 How is it that you fail to understand that I did not speak about bread? Beware of the leaven of the Pharisees and Sadducees.” 12 Then they understood that he did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees.

Just as in 12:38-40, the religious leaders come to Jesus and ask for a sign—they want proof, in other words. But what has Jesus been doing except giving sign after sign after sign? That’s his point about the red sky—“Folks, you can easily discern when a storm is coming—are you really unable to discern who I am?”

Here’s the truth: they don’t want to understand what’s happening, because that would require them to respond. So, they would rather keep Jesus at arms' length and pretend they don’t know who he is than have to repent and listen to him.

This is exactly the reason why so many people today claim not to believe—because if they actually admitted that Jesus is who he says he is, then they would have to live differently.
And the sign of Jonah? The early Church saw Jonah as a symbol for Christ:

Jonah was in the belly of the fish for 3 days; Jesus was in the belly of the earth for 3 days. Jonah was vomited up from death to life; Jesus was vomited up from death to life. Etc.

So, the Resurrection is the ultimate sign of the identity of Jesus. If we can’t see the empty tomb for what it is, then God help us.

P.S. The “leaven of the Pharisees and the Sadducees” is the false teaching that leads many people astray into unbelief.