Talmud for Nedarim 5:4
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 said to his neighbor:) "I am cherem to you" [Benefitting from me is forbidden to you as cherem (dedicated property) — you may not benefit from me], the bevowed one is forbidden [even if he did not answer "Amen"; for one may forbid his friend from benefitting from him.] (If he said:) "You are cherem to me," [forbidding to himself all benefit from his neighbor], the vower is forbidden. "I am (cherem) to you, and you are (cherem) to me," they are both forbidden. And they are both permitted with what appertains to those who come up from Bavel [such as a well for those who come up for the festivals. For they would come up from Bavel to Eretz Yisrael for the festivals. The well was in the middle of the way, and all of Israel could use it, being as hefker (ownerless), and not as something jointly owned.], and they are forbidden with what appertains to that city.