Shaykh Ibn ‘Uthaymeen explains that the default ruling is that all water is pure (tahur) and can be used for purification unless proven otherwise.

The basis for the ruling on stagnant water is the hadith of Abu Hurayrah, in which the Prophet (peace and blessings of Allah be upon him) forbade urinating in stagnant water and then performing ghusl from it. The Shaykh clarifies that the prohibition is on the action of contaminating the water, not necessarily on the water itself in all cases.

He differentiates between a large and small body of stagnant water. If the water is abundant (like a large pond or lake), the addition of a small amount of impurity does not render it impure, provided none of its three properties—color, taste, or smell—have changed. This is based on the principle that a large amount of water is not easily affected by filth. Therefore, wudu from such water would be valid.

However, if the body of stagnant water is small, urinating in it would render it impure (najis) because the impurity would overwhelm the water. In this state, it cannot be used for purification.

The wisdom behind the prohibition is to prevent people from contaminating a limited water source that others may need and to protect oneself from using water that has become impure. Therefore, if one encounters stagnant water that has not been altered by impurities, it is considered pure and permissible to use for wudu and ghusl.