Responsa hatam Sofer, Orah hayim 36

17

Indeed, in the countries where we reside, where the gentile women walk about bareheaded, but our mothers did not go out like that and were most careful on that score, concerned as they were about the words of the Zohar on this subject. They were extremely particular in this regard despite the fact that, had we been standing by in the capacity of a religious quorum to fix the halakhah, we would have said that that rule of conduct is interpreted in the Talmud in a permissive manner—that is to say, pursuant to the interpretation of the Arukh [ . . . ] and that the halakhah is not in accordance with the Zohar. Nonetheless, since our mothers adopted the custom as mentioned in the Zohar, our Teacher, R. Elijah [Stein], has written in relation to this sort of thing that even a foolish custom can uproot a halakhah, and itself attain the status of a fixed halakhah! That is to say, where external works dissent from the view of the Talmud, and by “external works” is meant tractate Soferim and the like, or a Midrashic work or the Pesikta—and the Zohar falls into a similar category; such a custom has the effect of uprooting the halakhah and becomes a halakhah amongst Israel.