אמר לעדים בואו והעידוני שבועה שלא נעידך נשבע לבטל את המצוה שלא לעשות סוכה ושלא ליטול לולב ושלא להניח תפילין זו היא שבועת שוא שחייבין על זדונה מכות ועל שגגתה פטור
GEMARA. Ulla said: Provided that it was already known to three men. IF HE SWORE CONCERNING A THING WHICH IS IMPOSSIBLE, [AS E.G., ] 'IF I HAVE NOT SEEN A CAMEL FLYING IN THE AIR.' T swear that I have seen,' he does not say! What [then] is meant by, 'If I have not seen ? Abaye said: Learn, I SWEAR I have seen.' Raba said: [The Mishnah means:] he said, '[I swear that] all the fruits of the world shall be prohibited to me, if I have not seen a camel flying in the air.' Said Rabina to R. Ashi: Perhaps this man saw a large bird, and gave it the name of camel, and when he swore, he swore according to his own mind; and if you say, we go according to his mouth, and we do not go according to his mind, [that cannot be,] for it has been taught: When they adjure him, they say to him, 'Know that we do not adjure you according to your own mind, but according to the mind of the Omnipresent and the mind of the Beth din.' What is the reason? Is it not because we say, perhaps he gave him counters, and called them zuzim, in which case when he swears, he swears according to his own mind? - No! There [the reason is] because of the cane of Raba. <br>
Sefer HaChinukh
To not swear in vain: That we not swear pointlessly, as it is stated (Exodus 20:7), "You shall not take the name of the Lord, your God, in vain." And the notion of pointlessness has four angles: [The first is,] for example (Shevuot 29a), that he swears about something known [to be true] that it is not so, like swearing about a pillar of marble that it is a pillar of gold. And so [too], anything like this. The second angle is, for example [Talmud Yerushalmi Shevuot 3:8], that he swears about what is known to be so, like about a stone that it is a stone and about a tree that it is a tree, and all that is like this. The third angle is that he swears to negate this commandment or the commandments that God, blessed be He, commanded us; as this is also completely pointless, since it is not in his hand to swear [to negate] that which God has already obligated him - and it is like the one who swears about something known that it is not so. The fourth angle is that he swears to do something that he does not have the power to do; for example (Shevuot 25a) that he will not sleep for three consecutive days, or that he will not eat for seven consecutive days. And so [too], anything like this.
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Sefer HaMitzvot
That He prohibited us - that we not make a vain oath. And that is His, may He be blessed, saying, "You shall not take the name of the Lord in vain" (Exodus 20:7). And that is that one swears about something - the existence of which is axiomatic - that it is different than what it actually is, or about the existence of something impossible or [if] he swears to negate the matter of a commandment. And likewise if he swears about [the truth of] something known, that no one denies or disagrees about at all - for example, he swears by God that anything that is slaughtered, dies - this one has also taken the Lord's name in vain. (Yerushalmi Shevuot 3) And the language of the Mishnah (Shevuot 29a) is, "Which is a vain oath? One who swears to change that which is known." And one who transgresses this negative commandment is lashed if he was volitional, and exempted if he was inadvertent - like the other ones guilty of [violation of] a negative commandment, as we explained. And there, they said in Shevuot (Shevuot 29a), "This is the vain oath on an utterance for which one is liable for lashes, when volitional; and exempted when inadvertent." And the regulations of this commandment are explained there. (See Parashat Yitro; Mishneh Torah, Oaths 1.)
