Matthew 16:1-4

Now when the Pharisees and Sadducees came to test Jesus, they asked him to show them a sign from heaven. He said, “When evening comes you say, ‘It will be fair weather, because the sky is red,’ and in the morning, ‘It will be stormy today, because the sky is red and darkening.’ You know how to judge correctly the appearance of the sky, but you cannot evaluate the signs of the times. A wicked and adulterous generation asks for a sign, but no sign will be given to it except the sign of Jonah.” Then he left them and went away.

Matthew 12:40-42 provides a bit more context, “For as Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth. The men of Nineveh will rise up in the judgment with this generation and condemn it, because they repented at the preaching of Jonah; and indeed a greater than Jonah is here. The queen of the South will rise up in the judgment with this generation and condemn it, for she came from the ends of the earth to hear the wisdom of Solomon; and indeed a greater than Solomon is here.” The Pharisees want a sign because, in part, they want Jesus to be what they need Him to be to justify what they believe and what they teach. They seek only what Jesus can do for them, rather than seeing Jesus as the bread of life. He is the life, not the signs.