Reference for Shevuot 3:8
אֵיזוֹ הִיא שְׁבוּעַת שָׁוְא, נִשְׁבַּע לְשַׁנּוֹת אֶת הַיָּדוּעַ לָאָדָם, אָמַר עַל הָעַמּוּד שֶׁל אֶבֶן שֶׁהוּא שֶׁל זָהָב, וְעַל הָאִישׁ שֶׁהוּא אִשָּׁה, וְעַל הָאִשָּׁה שֶׁהִיא אִישׁ. נִשְׁבַּע עַל דָּבָר שֶׁאִי אֶפְשָׁר, אִם לֹא רָאִיתִי גָמָל שֶׁפּוֹרֵחַ בָּאֲוִיר, וְאִם לֹא רָאִיתִי נָחָשׁ כְּקוֹרַת בֵּית הַבַּד. אָמַר לְעֵדִים בֹּאוּ וַהֲעִידוּנִי, שְׁבוּעָה שֶׁלֹּא נְעִידֶךָ. נִשְׁבַּע לְבַטֵּל אֶת הַמִּצְוָה, שֶׁלֹּא לַעֲשׂוֹת סֻכָּה, וְשֶׁלֹּא לִטֹּל לוּלָב, וְשֶׁלֹּא לְהָנִיחַ תְּפִלִּין, זוֹ הִיא שְׁבוּעַת שָׁוְא, שֶׁחַיָּבִין עַל זְדוֹנָהּ מַכּוֹת וְעַל שִׁגְגָתָהּ פָּטוּר:
Which is a vain oath? Swearing to what is at variance with what is known to man: saying about a pillar of stone that it is gold; about a man, that he is a woman; about a woman, that she is a man. Swearing to the impossible: If I did not see a camel flying in the air. [i.e., Let all the fruits in the world be forbidden to me if I did not see a camel flying in the air] and if I did not see a snake like the beam of the olive press. [i.e., in the shape of the beam of the olive press. For (if he said) as thick as the beam of the olive press, this would not be a vain oath, there being many such.] If he said to witnesses: "Come and testify for me," (and they said:) "We swear that we will not testify for you," (this is a vain oath). [This is negating a mitzvah, for they are obliged to testify, it being written(Leviticus 5:1): "If he does not tell (i.e., testify), then he shall bear his sin."] Swearing not to perform a mitzvah: not to make a succah, not to take a lulav, not to put on tefillin. This is a vain oath, where for willful transgression one is liable to stripes, and where for unwitting transgression, he is not liable.