Mark 8:12
And he sighed deeply in his spirit, and saith, Why doth this generation seek after a sign? verily I say unto you, There shall no sign be given unto this generation. And he sighed deeply in his spirit, and saith, Why doth this generation seek after a sign? verily I say unto you, There shall no sign be given unto this generation.
The deep sigh was grief over a generation that kept demanding a sign despite the evidence already given, the demand coming from the Pharisees and Sadducees for another heavenly sign. No further outward miracle would be given because external evidence and intellectual proof could not meet their need, which was spiritual renewal rather than more proof.
Signs from heaven had already attested the mission, and the preaching itself, spoken with the convicting power of the Holy Spirit, was already God's sign for their salvation to that generation, just as Jonah's preaching was a sign to Nineveh. The only sign given was the sign of Jonah, pointing to Christ being in the heart of the earth for three days and nights, as Jonah was in the great fish for that time. Because Nineveh repented at Jonah's warning, its people will condemn those who rejected the greater message of Christ.