Matthew 16:1-4

And the Pharisees and Sadducees came, and to test him they asked him to show them a sign from heaven. He answered them, “When it is evening, you say, ‘It will be fair weather, for the sky is red.’ 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. 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.

Pharisees and Sadducees - "It must have been startling to those who knew them to see these men together at all, and together on a matter of religious principle, and on a mission to a religious teacher. The Pharisees were the traditionalists of their day. They believed in God, they believed in the work of the Holy Spirit. They believed in the essential purity and holiness of God. They believed in the fundamental things. They believed in angels; they believed in resurrection; they held all the spiritual verities; but they hid them from the people whom they were supposed to teach and lead … under the grave-clothes of ritualism and tradition. The Sadducees were the rationalists of their day. They did not believe in spirits, they did not believe in angels, they did not believe in resurrection. They denied all supernatural elements in religion. They made religion a mere ethical code. The Pharisee had no dealings with the Sadducee; the Sadducee looked with supreme contempt on the Pharisee. The Sadducees had left Jesus alone up to this point. Men absolutely divided as between themselves were united on this occasion … The Pharisees would say, This thing can be, there can be signs out of heaven, but He cannot produce them; therefore let us ask Him to do it. The Sadducees would say, There could be no sign out of heaven, so you are safe in asking Him to do it. We have two opposing ideas, but united in their opposition to Him. And so in a common hostility to Him, a common desire to see His defeat, these men came to Him. Jesus Christ did not say these men were wrong. They came to criticize Him, but they went away criticized; they came to measure Him, but they went away measured." (Morgan)

test him - they did not believe He could do what they asked
adulterous generation...sign of Jonah - adulterous = faithless, treacherous

"The nation was unfaithful in its vows to the Lord.  Here and in Luke 11:29-32 the sign is the warning of judgment to come (cf. Jonah 1:2; 3:4).  In Matthew 12:39 the sign is related to the death and resurrection of the Son of Man." (Ryrie)

"Being familiar with the prophets they knew that certain signs had been indicated therein which were to take place before the manifestation of the Messiah; so they came to Jesus, without any desire to know the truth, but simply as tempting or testing Him, asking that He show them a sign from heaven. They meant a sign indicating that the Messianic age was a hand. Jesus rebuked them for their unbelief. They were quite able to read the signs of the heavens in regard to matters of weather or climatic conditions, but they were absolutely unable to discern the signs of the times. Had their eyes been opened they would have realized that all the miraculous works of Jesus were in themselves the signs of the age to come and told of the presence of the King. Messiah was in their midst. No other sign would be given to them until the sign of the prophet Jonah." (Ironside)