Chapter on Water and Its Rules, and What is Permissible and Impermissible for Purification

[4] Leftover water (su’r) – this is the water inside the vessel that remains as ‘leftover’ after having been used by a person, such as by dipping their hands into this vessel to draw water. In this example, the leftover water of a menstruating woman is not inherently impure. However, there is an implicit precaution: if there is a reason to believe the water may have been contaminated (for example, if her hands were impure when she dipped them into the vessel), then caution should be exercised