Mishnah Sheviit 7:4משנה שביעית ז׳:ד׳
If one buys a first-born animal for his son’s [wedding] feast, or for a festival, and then decides that he has no need for it, he may sell it. Trappers of wild animals, birds and fishes, who chanced upon unclean species, may sell them. Rabbi Judah says: also a man who happened to chance one upon by accident may buy or sell, provided that he does not make this into his regular profession. But the sages prohibit.
Tosafot on Pesachim 23a:10:1תוספות על פסחים כ״ג א:י׳:א׳
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Penei Yehoshua on Pesachim 23a:5פני יהושע על פסחים כ״ג א:ה׳
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Haamek Davar on Genesis 2:17:1העמק דבר על בראשית ב׳:י״ז:א׳
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Arukh HaShulchan, Orach Chaim 117:1ערוך השולחן, אורח חיים קי״ז:א׳
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Arukh HaShulchan, Yoreh De'ah 117:1ערוך השולחן, יורה דעה קי״ז:א׳
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Arukh HaShulchan, Yoreh De'ah 117:2ערוך השולחן, יורה דעה קי״ז:ב׳
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Arukh HaShulchan, Yoreh De'ah 117:3ערוך השולחן, יורה דעה קי״ז:ג׳
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Arukh HaShulchan, Yoreh De'ah 117:4ערוך השולחן, יורה דעה קי״ז:ד׳
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