Nedarim5: 4
,,הֲרֵינִי עָלֶיךָ חֵרֶם” — הַמֻּדָּר אָסוּר; ,,הֲרֵי אַתְּ עָלַי חֵרֶם” — הַנּוֹדֵר אָסוּר; ,,הֲרֵינִי עָלֶיךָ וְאַתְּ עָלַי” — שְׁנֵיהֶם אֲסוּרִין. וּשְׁנֵיהֶם מֻתָּרִין בְּדָבָר שֶׁל עוֹלֵי בָבֶל, וַאֲסוּרִין בְּדָבָר שֶׁל אוֹתָהּ הָעִיר.
[If one says:] “I am to you cherem,” the person prohibited by the neder is forbidden; “You are to me cherem” —- the person making the neder is forbidden; “I am to you and you are to me” —- both are forbidden. Both are permitted in something of the pilgrims from Babylon, but are forbidden in something of that city.