Midrash for Chullin 267:34
דתניא ר' יהודה בן אגרא אומר משום רבי מאיר
The other [Resh Lakish] retorted: Teach it even in Ben Taddal's<span class="x" onmousemove="('comment',' A fictitious name for some foolish babbler (Jast.) . Variants are: a stammerer, cf. Ex. VI, 12; probably names of persons known to have been unreliable in all matters.');"><sup>25</sup></span>
Sifrei Devarim
"from the slaughterers of the slaughtering": The time of the slaughtering is the criterion. From here they ruled: If a stranger converted and he had a cow — If it were slaughtered before he converted, he is exempt (from priestly gifts); if after he converted, he is liable. If the matter is in doubt (safek), he is exempt, and "the burden of the proof is upon him who would remove (the object) from his neighbor's domain.
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