Reference for Chullin 279:31

אלא לשילוח

If for the one that must be set free, but surely the Torah would not enjoin to set it free if it would thereby lead to transgression!<span class="x" onmousemove="('comment',' Lit., 'for a stumbling-block'. The finder of the bird, not knowing that it originally came from a beguiled city, will eat it, and so be led into sin by another's performance of a precept. On this ground therefore it cannot be suggested that birds from a beguiled city may be used for the leper's offering.');">²²</span>

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