[Avodah] Poletet shichvat zera, and shiva neki'im

[Ken Bloom]

I'm learning the halachot about the shiva neki'im and the person I'm
learning with taught me about how pelitat shichvat zera is soter a day of
the shiva neki'im, necessitating one to count 4 or 5 days before hefsek
tahora. The person I'm learning with has been very good about giving me
simple explanations of the torah sources for most of the halachot, but all
he could say about this one is that it's some sort of derasha that he
couldn't remember. My books also don't quote a paseuk or derasha for this,
even though they do in other places where it's relevant.

Can anyone holding in Daf Yomi right now, or otherwise versed in the
subject, tell me what the soruce is for pelitat shichvat zera being soter a
day of the shiva neki'im?
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