Reference for Bekhorot 66:16

יקיז אע"פ שהוא עושה בו מום:

<big><b>GEMARA: </b></big>Our Rabbis taught: We may let blood of a firstling which had an attack of congestion, in a part [of the body] where it is not made blemished, but we must not let blood in a part [of the body] where a blemish is caused.

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