Reference for Beitzah 4:14

רב יוסף אמר גזרה משום פירות הנושרין

Rav Yoseif says: "[The egg is forbidden in our Mishnah because of] a rabbinic decree because of the fallen fruits." [I.e. just as all fallen fruits that you find on the ground are forbidden on Yom Tov – in case they fell on Yom Tov – so too is an egg that you find on Yom Tov is forbidden in case it was laid on Yom Tov – and thus all eggs laid on Yom Tov are forbidden.]

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