Midrash for Chullin 267:51
קרבן אשתו וחלה ובכור בהמה טמאה ובכור בהמה טהורה לחיוב
the firstling of an unclean animal,<span class="x" onmousemove="('comment',' Where there was a doubt whether the proselyte's ass brought forth a firstling before his conversion or after. If after, then the foal is forbidden for all purposes until it is redeemed with a lamb (cf. Ex. XIII, 13) , which lamb had to be given to the priest; in this case of doubt, the proselyte must redeem the foal with a lamb, but he may withhold it from the priest; v. infra n. 20.');"><sup>38</sup></span> and the firstling of a clean animal,<span class="x" onmousemove="('comment',' The doubt here as in prec. note. This case of doubt may involve the penalty of kareth for slaughtering a firstling outside the Temple.');"><sup>39</sup></span> he is held liable;<span class="x" onmousemove="('comment',' Since these cases are matters which involve religious prohibitions and entail serious penalties, we must adopt the stricter view and impose the obligation upon the proselyte.');"><sup>40</sup></span>