When does stagnant water become ritually impure?

Question:
When does stagnant water become ritually impure?

Answer:
When it is changed by ritual impurity, or when it is very little.
Q: If one of its three characteristics is changed?
The Shaykh: If it is changed, it becomes ritually impure, even if it is a large amount. And if it is a small amount...
Q: And if it is less than two qullahs?
The Shaykh: It is a matter of scholarly disagreement. It may become ritually impure, or it may not, according to the correct view.
Q: What is the correct criterion?
The Shaykh: Some scholars hold that water less than two qullahs becomes ritually impure merely by contact with impurity. However, the correct view is that if it is a large amount, it does not become ritually impure unless it is changed. As for a small amount in a vessel, as the Prophet (peace and blessings be upon him) said, if a dog drinks from it, pour it out; because in small vessels, impurity usually affects the water; therefore, it is obligatory to pour it out.