Abu Yusuf is reported to have used movement produced by the hand as the criterion.
The reason is that stirring by hand is a minimal action for detecting whether movement travels across the water.
This disagreement concerns how to measure the extent of the pond, not whether impurity itself is legally significant.
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Impurity in Stagnant and Running Water, Large Ponds, and Aquatic Creatures
Abu Yusuf is reported to have used movement produced by the hand as the criterion.
The reason is that stirring by hand is a minimal action for detecting whether movement travels across the water.
This disagreement concerns how to measure the extent of the pond, not whether impurity itself is legally significant.