NUM350 "That which is gone out of thy lips, Thou shalt observe and do…"; "… he shall do according to all that proceeded out of his mouth." (Deuteronomy 23:24; this verse]. Although both of these passages are parts of commandments that seem to be talking about formal vows made to God, Maimonides sees in these imperatives a general obligation to keep one's promises to one's fellow man, honor one's word and carrying out one's pledge even if expressed in an informal way (Sefer ha-Mitzvot le-Rambam, positive command 94; see comments of Ramban. See also Sefer ha-Chinnukh, mitzvot 407 and 575).