Reference for Bekhorot 81:42

(שמות ט, י) ויהי שחין

is where it is dry inside and moist outside, for R'Joshua B'Levi said: The boil which the Holy One, blessed be He, brought upon the Egyptians was moist outside and dry inside, for it is written: And it became a boil breaking forth with blains upon man and upon beast.<span class="x" onmousemove="('comment',' Ex. IX, 10 The word in the text is connected with 'pouring forth', implying something wet and moist.');">²⁶</span>

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