A view attributed to Zufar distinguishes water used by someone already ritually pure from water used to remove hadath. In the former case the water remains purifying; in the latter it is pure but no longer purifying.
The basis is whether the water actually removed ritual impurity, rather than treating all devotional washing alike.
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Used Water: Definition, Ruling and Juristic Differences
A view attributed to Zufar distinguishes water used by someone already ritually pure from water used to remove hadath. In the former case the water remains purifying; in the latter it is pure but no longer purifying.
The basis is whether the water actually removed ritual impurity, rather than treating all devotional washing alike.