Reference for Bekhorot 33:17

אתא רב אושעיא מנהרדעא ואייתי מתניתא בידיה

[Hardly so], for does he not hold with the Tanna of the School of Ishmael who taught: Lambs whose wool is hard, are exempt from the rule of the first shorn wool, for it says: And if he were not warmed with the fleece of my sheep?<span class="x" onmousemove="('comment',' Job. XXXI, 20. Therefore only fleece that warms is called fleece, and the fleece of a goat born of a ewe is hard, goat's wool being hard; v. Hul. 137a.');">⁹</span>

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