Rif Pesachim 17a {Pesachim 50b continues ... 51a; Yerushalmi}

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17a

{Pesachim 50b continues}
and it is written {Tehillim 57:11}

יא כִּי-גָדֹל עַד-שָׁמַיִם חַסְדֶּךָ; וְעַד-שְׁחָקִים אֲמִתֶּךָ.

11 For Thy mercy is great unto the heavens, and Thy truth unto the skies.

{Thus על vs. עד = above vs. until}
How so?
Here for those who perform {Hashem's command} for its own sake, and here for those who perform not for its own sake {but for ulterior motives}, as Rav Yehuda cited Rav.
For Rav Yehuda cited Rav: One should always engage in Torah and mitzvot even not for its own sake, for out of doing it not for its own sake, he will {eventually} come to do it for its own sake.

The residents of Beyshan were accustomed not to travel from Tzor to Tzidon on erev Shabbat. Their children came before Rabbi Yochanan.
They said to him: for our fathers it was possible not to come, and so they did not come. For us it is not possible. What {is the law}?
He said to them: your fathers already accepted this on themselves, and it is written {Mishlei 1:8}:

ח שְׁמַע בְּנִי, מוּסַר אָבִיךָ; וְאַל-תִּטֹּשׁ, תּוֹרַת אִמֶּךָ.

8 Hear, my son, the instruction of thy father, and forsake not the teaching of thy mother;

They learnt {in a brayta}: Things which are permitted, yet others treat them as forbidden, {Pesachim 51a} you are not permitted in their presence.

Rav Chisda said: This refers to Cutheans. What is the reason? For they confound and compare one thing with another, and will come to permit the forbidden.
And not just Cutheans is it forbidden to permit for, but any place where they are not Torah-knowledgeable.
As they learnt {in a brayta}: Two brothers may bathe together, yet two brothers may not bathe together in Cabul. And {?For?} there was an incident involving Yehuda and Hillel, the sons of Rabban Gamliel, where both of them bathed at once in Cabul, and the entire region criticized them, saying, "In our days we have not seen such," whereupon Hillel slipped away and went out into the outer chamber, but he did not wish to inform them that they were permitted to do this, for they learnt {in a brayta}: A person may bathe with anyone, with the exception of his father, his father-in-law, and his sister's husband. Rabbi Yehuda permits with his father because of his father's honor.
And it was taught {tana}: A student should not bathe with his master, and if his master requires him, it is permitted.

Yerushalmi: Rabbi Eleazar beRabbi Bon says: Any matter which he does not know is permitted and he errs and acts as if it is forbidden, and he asks {to nullify this minhag} they permit for him. And any matter which he knows is permitted and conducts himself as if it is forbidden, and he asks {to nullify the custom he adopted} they do not permit for him.
All sorts of matters we ascribe to be {a valid} custom. Women who are accustomed not to do work on Rosh Chodesh, this is a custom. And not to do work on motzaei Shabbat until the Bet Midrash lets out, it is a minhag. And on Monday and Thursday until the fast is done, it is a minhag.

{Gloss: And not to do work on motzaei Shabbat - it is not a minhag. Until the Bet Midrash lets out, it is a minhag. And on Monday and Thursday - it is not a minhag. Until the Bet Midrash lets out, it is a minhag.}