Mark 8:11-13

11 The Pharisees came up and started a discussion with him; they demanded of him a sign from heaven, to put him to the test.

12 And with a profound sigh he said, 'Why does this generation demand a sign? In truth I tell you, no sign shall be given to this generation.'

13 And, leaving them again, he re-embarked and went away to the other side.