Luke 24:25
And he said unto them, O foolish men, and slow of heart to believe in all that the prophets have spoken!
Mark 7:18
And he saith unto them, Are ye so without understanding also? Perceive ye not, that whatsoever from without goeth into the man, it cannot defile him;
Mark 8:17-18
And Jesus perceiving it saith unto them, Why reason ye, because ye have no bread? do ye not yet perceive, neither understand? have ye your heart hardened?
Mark 9:19
And he answereth them and saith, O faithless generation, how long shall I be with you? how long shall I bear with you? bring him unto me.
Mark 16:14
And afterward he was manifested unto the eleven themselves as they sat at meat; and he upbraided them with their unbelief and hardness of heart, because they believed not them that had seen him after he was risen.
Hebrews 5:11-12
Of whom we have many things to say, and hard of interpretation, seeing ye are become dull of hearing.