Reference for Chullin 281:22

עשה ולא תעשה הוא ואין עשה דוחה לא תעשה ועשה

Now he has already transgressed the negative precept, and there remains only the positive precept; and one might suppose that now a positive precept can override this [remaining] positive precept,<span class="x" onmousemove="('comment',' I.e., that the positive precept of offering birds for the leper's sacrifice should override the positive precept of letting the dam go.');">¹⁵</span>

Rashi on Deuteronomy

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