Luke 24:25
Then He said to them, "O men sluggish in mind and slow in heart to believe all that the prophets have said!
Mark 7:18
And He answered them, "Are you too without understanding yet? Do you not know that nothing from the outside that goes into a man can make him foul,
Mark 8:17-18
And as He noticed it He said to them, "Why are you discussing the fact that you have no bread? Do you not yet know nor understand? Are your minds so dull?
Mark 9:19
He answered them, "Oh, you unbelieving people of the times! How long must I be with you! How long must I put up with you! Bring him to me."
Mark 16:14
Later on He appeared to the Eleven themselves while they were at table, and reproved them for their lack of faith and their stubbornness, because they had not believed those who had seen Him after He had been raised from the dead.
Hebrews 5:11-12
I have much to say to you about Him, but it is difficult to make it clear to you, since you have become so dull in your spiritual senses.