The 157th prohibition is that we are forbidden from violating any verbal obligation we have made, even if it was not said as an oath.
These obligations are known as nedarim (vows); for example, when a person says, "if a certain event occurs" or "if I do a certain act" then "all fruit will be forbidden to me" or "the fruit of this country [will be forbidden to me]" or a certain food, such as milk, fish, etc. "will be forbidden to me"; or when he says, "deriving pleasure from my wife is forbidden to me"; or any similar verbal obligation, as explained in tractate Nedarim. In all these cases he must carry out his vow, and violating it counts as a prohibition.
The source of this commandment is G‑d's statement,1 "He must not yachel ("break") his word. He must do everything that he stated verbally."
Our Sages explained the phrase, "He must not yachel his word" as meaning, "he must not make his word profane (chullin)," i.e., to obligate himself and then not fulfill his promise.2 As tractate Shavuos puts it, "vows comes under the prohibition, 'he must not break (yachel) his word.' "
The Sifra3 says [regarding a case where someone promised to bring a sacrifice and did not do so], "The verse, 'He must not break' teaches us that he violates the prohibition of not breaking one's word and that of not delaying as offering." This means that if a person vowed to bring a sacrifice, and three holidays have passed by and he still did not do so, then he is guilty of transgressing the prohibition of not delaying [the offering]4 and of not breaking his word.
The same applies to anything resembling a sacrifice, such as promising a gift to the fund of the Holy Temple,5 to charity, to a synagogue, etc.
One who transgresses this prohibition by doing something he has promised not to, is punished by lashes.
The details of this mitzvah are completely explained in tractate Nedarim.
