Reference for Yoma 154:14

אמר אביי שאני נחל דרדיפי מיא

Some say: He permitted them to go and to return [through water], whereupon Abaye said: Quite right [to permit them] to do so on the way to the lecture, but why the permission on their return? - Lest you put a stumbling-block in their way for the future. Rab Judah and R'Samuel son of R'Judah were standing at the bank of Nehar Papa,<span class="x" onmousemove="('comment',' [A canal that passed through Pumbeditha, v. Obermeyer. p. 227].');">¹⁴</span>

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