Reference for Bava Batra 42:4

אמר רב הונא האי בר מבואה דאוקי ריחיא ואתא בר מבואה חבריה וקמוקי גביה דינא הוא דמעכב עילויה דא"ל קא פסקת ליה לחיותי

He [Joab] then went to his teacher and asked: How didst thou teach me to read? He replied: Zeker. Thereupon he drew his sword and threatened to kill him. Why do you do this? asked the other. He replied: Because it is written, Cursed be he that doeth the work of the Lord negligently.<span class="x" onmousemove="('comment',' Jer. XLVIII. 20, The 'negligence' consisted in the fact that his teacher had allowed him when a boy to read zekar without correcting him (v. Tosaf.). ');">⁴</span>

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