Maimonides on the Laws and Fulfillment of Vows

(viii) Maimonides, Opening to Laws of Vows:
“They contain 3 mitzvot: two positive commandments and one negative commandment. They are:
1. To heed the utterances of one’s mouth and to carry out one’s vow;
2. Not to desecrate one’s word;
3. To nullify a vow or an oath; this is the law concerning the nullification of oaths explicitly stated in the Torah.
These mitzvot are explained in the ensuing chapters.”

(ix) Maimonides, Counting of the Mitzvot, Positive Commandment 94:
“For a person to fulfill all which he gives out with his lips (i.e. utters), be it a sacrifice, (a gift to) charity, or the likes, as it is stated (-Deuteronomy 23:2): “What you have given out with your lips (i.e. spoken), take heed and fulfill....’”