Luke 24:25
He said to them: You are foolish men. You are slow of heart to believe in all that the prophets have spoken!
Mark 7:18
He replied: Do you not understand? Do you not know that which goes into the man from the outside cannot defile him?
Mark 8:17-18
Hearing this Jesus said: Why do you reason about having no bread? Do you not understand? Are your hearts dull of understanding?
Mark 9:19
Jesus then said: O faithless generation. How long will I be with you? How long must I put up with you? Bring him to me.
Mark 16:14
Afterward he was revealed to the eleven as they sat at the table. He rebuked them for their unbelief and hardness of heart, because they did not believe those who had seen him after he had risen.
Hebrews 5:11-12
We have many things to say about him and it is hard to explain it to you, for you are dull of hearing (slow to learn).