The Shulchan Aruch · Yoreh De'ah
יורה דעה ר״טYoreh De'ah 209
Siman 209 of 402 in Yoreh De'ah — 1 se'if
Se’if 1א׳
אֶחָד הַנּוֹדֵר מֵעַצְמוֹ אוֹ שֶׁהִדִּירוֹ חֲבֵרוֹ וְאָמַר: אָמֵן, אוֹ דָּבָר שֶׁעִנְיָנוֹ כַּעֲנִיַּת אָמֵן שֶׁהוּא קַבָּלַת דְּבָרִים, אָסוּר; אֲבָל כְּשֶׁחֲבֵרוֹ מַדִּירוֹ מִשֶּׁלּוֹ אֵין צָרִיךְ לְקַבֵּל דְּבָרָיו.
It is all one whether he vows of himself or his fellow imposed the vow upon him and he said: Amen, or a thing whose sense is like the answering of Amen, which is an acceptance of the words — he is forbidden. But when his fellow imposes a vow upon him from what is his own, he need not accept his words.
Shulchan Aruch, Yoreh De'ah — Torat Emet 357 edition, via Sefaria — public domain
English rendered from the Hebrew for this app — The Gematria's own rendering program. A rendering of the words, not a ruling; practical halacha is learned with a rav