Reference for Keritot 50:11

אלא אומרים לו המתן עד שתכנס לבית הספק

Raba also said: What is the reason of the Rabbis who maintain that one may not bring a suspensive guilt-offering except for a [particular] sin the wilful transgression of which is subject to kareth and the inadvertent transgression of which is subject to a sin-offering?

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