The 94th mitzvah is that we are commanded to fulfill every verbal obligation we have taken upon ourselves, whether an oath, a vow, a korban,1 etc.
The source of this commandment is G‑d's statement,2 "You shall be careful to carry out whatever you say."
Although our Sages have split up this verse and explained each word separately as referring to something different, the general meaning of everything they say is: it is a positive commandment for a person to fulfill every verbal obligation he has taken upon himself, and a prohibition not to do so. This will be explained in our discussion of the prohibitions.3
The Sifri says, "The verse, 'Whatever you say,' constitutes a positive commandment." You find it obvious that the words, "whatever you say" by themselves have no meaning whatsoever. The intention [of the Sifri] is therefore as I explained above — that the plain meaning of the verse is that a person is obligated to carry out his verbal commitments.
This commandment is stated a second time in G‑d's statement,4 "A person must fulfill all he has verbally said."
The details of this mitzvah — i.e., how exactly the person must fulfill his obligation, and how he can be released [from his obligation] when in doubt regarding his statement — are explained in a number of passages in Sh'vuos, Nedarim, the end of Menachos, as well as in Kinim.
