Midrash for Chullin 267:33
אל תקניטני שבלשון יחיד אני שונה אותה
for it has been taught: R'Judah B'Agra says in the name of R'Meir: Gleanings that are in doubt are deemed to be gleanings, forgotten sheaves that are in doubt are deemed to be forgotten sheaves, and corners of the field that are in doubt are deemed to be corners of the field.
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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