Jesus said, "A wicked and adulterous generation seeketh after a sign; and there shall no sign be given unto it, but the sign of the prophet Jonas.." A similar passage is Luke 11:29-30: "And while the crowds were thickly gathered together, He began to say, "And when the people were gathered thick together, he began to say, This is an evil generation: they seek a sign; and there shall no sign be given it, but the sign of Jonas the prophet. 30 For as Jonas was a sign unto the Ninevites, so shall also the Son of man be to this generation."
To whom was He speaking? The Sadducees and Pharisees who knew the Scripture thoroughly but refused to see Him for who He was: God's Son and the Messiah for whom they claimed to be waiting. They counted on their righteousness in the performance of the Torah to save them, yet they twisted it so it meant what they wanted. They were wicked and idolatrous, committing spiritual adultery and justifying it by their works.
So today: Modern Pharisees and Sadducees, people who claim to know Jesus, seek signs. They seek healing, prosperity, and miracles while still living the way they please and justifying themselves. But the only sign they will be given is that of Jonah: he was three days and three nights in the belly of the whale (great fish), then was vomited up to preach repentance to Nineveh. In like manner, Jesus came preaching repentance to us, and He spent three days and three nights in the heart of the earth (Matt 12:40) so we could be justified before God and spread the message of repentance and forgiveness to the world.
Do you seek signs from God? Do I? We must check ourselves! We may need to turn from our sin and seek His forgiveness instead!