Luke 24:25
And he said to them, "O foolish ones, and slow of heart to believe all that the prophets have spoken!
Mark 7:18
And he said to them, "Then are you also without understanding? Do you not see that whatever goes into a person from outside cannot defile him,
Mark 8:17-18
And Jesus, aware of this, said to them, "Why are you discussing the fact that you have no bread? Do you not yet perceive or understand? Are your hearts hardened?
Mark 9:19
And he answered them, "O faithless generation, how long am I to be with you? How long am I to bear with you? Bring him to me."
Mark 16:14
Afterward he appeared to the eleven themselves as they were reclining at table, and he rebuked them for their unbelief and hardness of heart, because they had not believed those who saw him after he had risen.
Hebrews 5:11-12
About this we have much to say, and it is hard to explain, since you have become dull of hearing.