Reference for Bekhorot 10:5

מוסיפין על המדות ואין מוסיפין יתר על שתות

Said R'Hanina: I asked [R'Eliezer] in the great School of Learning [Beth Hamidrash:] 'Why were the first-born of asses different from the first-born of horses and camels? ' - He replied: 'It is a decree of Scripture'.<span class="x" onmousemove="('comment',' There is no special reason for this differentiation.');">⁴</span>

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