Nedarim 16a
נדרים ט״ז א — ד"ה מַתְנִי׳ זֶה חוֹמֶר בַּשְּׁבוּעוֹת מִבַּנְּדָרִים
Nedarim 16a:11
When someone declares that performing a sukkah, taking a lulav, or donning phylacteries is konam (forbidden to him), a neder makes the items prohibited and the mitzvot cannot be performed until the vow is dissolved, whereas a shebuah permits them since one cannot swear to violate the mitzvot.
מַתְנִי׳ זֶה חוֹמֶר בַּשְּׁבוּעוֹת מִבַּנְּדָרִים, וְחוֹמֶר בַּנְּדָרִים מִבַּשְּׁבוּעוֹת. כֵּיצַד? אָמַר: ״קֻוֽנָּם סוּכָּה שֶׁאֲנִי עוֹשֶׂה״, ״לוּלָב שֶׁאֲנִי נוֹטֵל״, ״תְּפִילִּין שֶׁאֲנִי מַנִּיחַ״ — בַּנְּדָרִים אָסוּר, בַּשְּׁבוּעוֹת מוּתָּר, שֶׁאֵין נִשְׁבָּעִין לַעֲבוֹר עַל הַמִּצְוֹת.
MISHNA: This rule, that oaths can render actions, which do not have actual substance, either prohibited or obligatory, is a stringency of oaths vis-à-vis vows, which do not take effect with regard to matters that do not have actual substance. And there is also a stringency of vows vis-à-vis oaths. How so? With regard to one who said: Making a sukka is konam for me, or: Taking a lulav is konam for me, or: Donning phylacteries is konam for me, in the case of vows, the items are rendered forbidden, and he may not perform the mitzva until the vow is dissolved. However, in the case of similar oaths, these items are permitted, as one cannot take an oath to transgress the mitzvot.
