Reference for Bekhorot 6:3

והדתנן

'A ewe which gave birth to a species of a goat or a goat which gave birth to a species of a ewe, is exempt from the duty of the firstling'.<span class="x" onmousemove="('comment',' For Scripture says: Or the firstling of a goat. Num. XVIII, 17. Both the firstling and the mother must belong to the same species and class i.e. a goat.');">³</span>

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