Mark 9:42

9:42 “If anyone causes one of these little ones who believe in me to sin, it would be better for him to have a huge millstone ¹  tied around his neck and to be thrown into the sea.

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.

¹ tn Grk “the millstone of a donkey.” This refers to a large flat stone turned by a donkey in the process of grinding grain (BDAG 661 s.v. μύλος 2; L&N 7.68-69). The same term is used in the parallel account in Matt 18:6.