וַיְדַבֵּ֤ר מֹשֶׁה֙ אֶל־רָאשֵׁ֣י הַמַּטּ֔וֹת לִבְנֵ֥י יִשְׂרָאֵ֖ל לֵאמֹ֑ר זֶ֣ה הַדָּבָ֔ר אֲשֶׁ֖ר צִוָּ֥ה יְהוָֽה׃
Mishneh Torah, Negative Mitzvot
Not to transgress in matters wherein one has assumed an obligation, as it is said, “….he shall not break his word” (Num. 30:3).