Reference for Bekhorot 110:15

אבוה דשמואל עביד להו לבנאתיה מקוה ביומי ניסן ומפצי ביומי תשרי

The father of Samuel made a mikweh<span class="x" onmousemove="('comment',' A gathering of flowing waters for ritual immersion. Samuel would have one specially constructed as he would not allow them to bathe in the rivers in case the rain water dropping from the clouds and the melting snows were greater than the flowing waters.');">⁹</span>

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