Akuperma and midwesterner: It’s a Gemara in Yevamos in the 40s or 50s brought down in the Shukchan Aruch (maybe 56b?). Havlad Kasher Aval Pagum Lik’hunah.
Shmecdrik: You missed the point. Yes, major Rishonim hold that. However, the idea has long ago been Paskened and the Shittos nowadays that would even require a Giyur L’chumra to be Yotzei Rashi’s opinion (I think Tosfos has different opinions different places, if I recall correctly, but I’d have to go over them again) are almost nonexistent. Did you know that major Rishonim hold that it is possible to convert away from Judaism (the Me’iri and others quoting Shittas Hag’onim)? It doesn’t matter, because that issue was also resolved long ago and most Poskim wouldn’t even use that Shittah as a Tziruf to be Mattir an Agunah. Sometimes there are major Shittos, but they have been Paskened against so completely that we just aren’t Misyaches to them at all when dealing with Halachah L’ma’aseh.