Reference for Makkot 32:6

כי הא דההוא מקרי דרדקי דהוה פשע בינוקי אדריה רב אחא ואהדריה רבינא דלא אשתכח דדייק כוותיה:

Like by the schoolteacher that would hit the children. Rav Acha made a vow against him teaching, but Ravina annulled it, because there couldn't be found one as precise as he.

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