Reference for Shevuot 77:14

ואלא משום דחמירא ומי חמירא והתנן אלו הן קלות עשה ולא תעשה חוץ מלא תשא חמורות זו כריתות ומיתות ב"ד ולא תשא עמהן

Because it is said: They confirmed and accepted:<span class="x" onmousemove="('comment',' Est. IX, 27.');">²⁴</span> they confirmed what they had long ago accepted.<span class="x" onmousemove="('comment',' I.e., at Mount Sinai; for acceptance of laws was at Mount Sinai. The deduction is made because acceptance must come before confirmation. [For the whole passage cf. Tosaf. Sot. 7.]');">²⁵</span>

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