He was asked:

If a man bathes beside a pool or a stone basin in a bathhouse or elsewhere, and it is not full, then some water runs off his body back into the basin, does that water acquire the status of used water or not? Likewise, if a person in a state of major ritual impurity puts his hand into the water or the stone basin, does the water acquire that status or not?

He was also asked how much water is needed so that it does not acquire the status of used water when a person in a state of major ritual impurity bathes in it; and about a bowl placed on the bathhouse floor, with used water flowing over it, which is then used, without being washed, to scoop water from a stone basin that is not full. Please give us a ruling; may you be rewarded.

He replied:

Praise be to Allah. Water that splashes from the body of someone bathing or performing ablution into the vessel containing water for purification does not give its contents the status of used water. Likewise, when a person in a state of major ritual impurity dips his hand into a vessel or a stone basin that is not full, its water does not acquire that status.

As for the quantity of water that does not acquire the status of used water when a person in a state of major ritual impurity bathes in it: this is when it is a large quantity, amounting to two qullahs.