1 Moses told the leaders of the tribes concerning the Israelites, “This is what the Lord has commanded: 2 If a man makes a vow to the Lord or takes an oath of binding obligation on himself, he must not break his word, but must do whatever he has promised. 3 “If a young woman who is still living in her father’s house makes a vow to the Lord or places herself under an obligation, 4 and her father hears of her vow or the obligation to which she has pledged herself, and her father remains silent about her, then all her vows will stand, and every obligation to which she has pledged herself will stand.
31 “It was said, ‘Whoever divorces his wife must give her a legal document.’ 32 But I say to you that everyone who divorces his wife, except for immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery. 33 “Again, you have heard that it was said to an older generation, ‘Do not break an oath, but fulfill your vows to the Lord.’ 34 But I say to you, do not take oaths at all—not by heaven, because it is the throne of God, 35 not by earth because it is his footstool, and not by Jerusalem, because it is the city of the great King.
Notes and References
"... The next stage presented in the Sermon on the Mount is quite readily connected with the Temple, for it was the pre-eminent place of swearing oaths and making vows, often accompanied by offerings and oblations (Leviticus 22:18). In this section, Jesus gave instructions principally regarding oaths, not vows ... (Matthew 5:33–7). Several texts in the Old Testament sternly caution people against failing to perform the things they have solemnly sworn by God that they would do. Even more serious is the problem of not completing a votive offering that one has promised to pay to the holy place in reciprocation for the receipt of God’s blessing connected with their pledge. To Jesus’ audience, all this was emphatically clear: Either if “a man vows a vow (euxētai euchēn) to the Lord, or swears an oath (omosēi horkon) to bind himself by a pledge (horisētai horismōi), he shall not break his word; he shall do according to all that proceeds out of his mouth” (Numbers 30:2) ... One way to avoid the risk of nonperformance of an oath was simply to “swear not at all” (Matthew 5:34). But there were other options as well. Nonperformance was most grievous if God’s name or his holy things had been invoked when the oath was sworn, which meant that the Divine would be demeaned and his holiness would be compromised by any failure of the oath. Thus, a second way to avoid the risk of offending God in making oaths was simply not to invoke God’s name or his holy things when swearing an oath. And this is what the Sermon on the Mount recommends ..."
Welch, John W. The Sermon on the Mount in the Light of the Temple (pp. 98-100) Ashgate, 2009