In the halachos of Niddah, there is a term which I have come across not infrequently. The term is בכתמים שומעין להקל, which roughly translated means "in [the halachos of blood] stains we listen to the leniencies." The term seems to refer to a halachic principle that in these halachos we are more willing to be lenient like a minority opinion or an opinion which is not usually given such halachic weight. I first encountered this term in the Shach, then saw it in the Rema, and subsequently saw it in the Rosh. This is not a term which I have come across, as of yet, in any earlier source.
A search of Shas showed that the term does not appear in this form (H/T Shokhet). I am wondering where this term originates, and being that it is not from the Talmud, how the concept came to be accepted.