Luke 24:25
"Then he said unto them, O fools, and slow of heart to believe all that the prophets have spoken:"
Cross References (ranked by relevance)
Mark 7:18
And he saith unto them, Are ye so without understanding also? Do ye not perceive, that whatsoever thing from without entereth into the man, it cannot defile him;
Mark 9:19
He answereth him, and saith, O faithless generation, how long shall I be with you? how long shall I suffer you? bring him unto me.
Mark 16:14
Afterward he appeared unto the eleven as they sat at meat, and upbraided them with their unbelief and hardness of heart, because they believed not them which had seen him after he was risen.