Mark 10:5

10:5 But Jesus said to them, “He wrote this commandment for you because of your hard hearts. ¹ 

Mark 16:14

16:14 Then he appeared to the eleven themselves, while they were eating, and he rebuked them for their unbelief and hardness of heart, because they did not believe those who had seen him resurrected.

[10:5]  ¹ tn Grk “heart” (a collective singular).