The ruling on praying while tied to a stone contaminated with impurity

Question : Another example, a third example: a man ties a rope to a large stone. The stone is contaminated with impurity, and he ties the rope to it, then ties the rope to his hand or his belly. What do you say about his prayer?

Answer : It is valid, because the large stone does not move when he walks; it is he who can be dragged by it.