Reference for Chullin 62:8

מטבילין בראשין ואין מטבילין בכיפין שאין מטבילין באויר

And whence do we know that immersion is not allowed in the arch of the wave? - From [the following Baraitha] which was taught: Immersion is allowed at the edge [of the wave] but not in the arch of the wave, for immersion is not allowed in mid-air.

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