Muhammad is reported to have measured movement caused by performing wudu; some Hanafi references prefer his criterion.
The rationale is that assessing the quantity of water arises frequently in matters of wudu, so the movement caused by ordinary wudu provides a relevant practical test.
All three approaches consider whether impurity is likely to reach the opposite side, while differing on how the movement test should be conducted.
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Impurity in Stagnant and Running Water, Large Ponds, and Aquatic Creatures
Muhammad is reported to have measured movement caused by performing wudu; some Hanafi references prefer his criterion.
The rationale is that assessing the quantity of water arises frequently in matters of wudu, so the movement caused by ordinary wudu provides a relevant practical test.
All three approaches consider whether impurity is likely to reach the opposite side, while differing on how the movement test should be conducted.