|A mustard seed|
Our Rabbis taught: A man should not stand up to say Tefillah either immediately after trying a case or immediately after a [discussion on a point of]halachah;5 but he may do so after a halachic decision which admits of no discussion.6 What is an example of a halachic decision which admits of no discussion? — Abaye said: Such a one as the following of R. Zera; for R. Zera said:7 The daughters of Israel have undertaken to be so strict with themselves that if they see a drop of blood no bigger than a mustard seed they wait seven [clean] days after it.8 Raba said: A man may resort to a device with his produce and bring it into the house while still in its chaff9 so that his animal may eat of it without its being liable to tithe.10 Or, if you like, I can say, such as the following of R. Huna. For R. Huna said in the name of R. Zeiri:11 If a man lets blood in a consecrated animal, no benefit may he derived from it [the blood] and such benefit constitutes a trespass. The Rabbis followed the rule laid down in the Mishnah,12 R. Ashi that of the Baraitha.13I don't know that this particular example of Abaye would count as a halacha pesuka for me. Indeed, it could readily distract me from tefillah. See my discussion here, where I discuss how it may have been a grass-roots practice adopted from Zoroastrian practice.
Maybe no discussion because it is a description of a practice of bnos yisrael. But not necessarily that it was universally adopted. After all, later on that same daf in Niddah where that practice is first mentioned, we see:
Raba took R. Samuel out for a walk38 when he discoursed as follows: If a woman39 was in protracted labour40 for two days and on the third she miscarried she must wait seven clean days; he being of the opinion that the law relating to protracted labour41 does not apply to miscarriages and that it is impossible for the uterus42 to open without bleeding. Said R. Papa to Raba: What is the point in speaking of one who was in protracted labour for two days seeing that the same applies even where there was the minutest discharge, since R. Zera stated, The daughters of Israel have imposed upon themselves the restriction that even where they observe only a drop of blood of the size of a mustard seed they wait on account of it seven clean days? — The other replied: I am speaking to you of a prohibition,43 and you talk of a custom which applies only where the restriction has been adopted.44And this is Rava, a contemporary of Abaye, who seems to indicate that in his days, this minhag had not necessarily been universally adopted.