Mark 8: 11 - 13
11 The Pharisees came and began to argue with him, seeking from him a sign from heaven, to test him.
12 And he sighed deeply in his spirit, and said, "Why does this generation seek a sign? Truly, I say to you, no sign shall be given to this generation."
13 And he left them, and getting into the boat again he departed to the other side.

EWTN