“Moses spoke to the heads of the tribes of the children of Israel, saying: This is the matter that the Lord commanded:” (Numbers 30:2). “If a man takes a vow to the Lord, or takes an oath to impose a prohibition upon himself, he shall not profane his word; he shall act in accordance with everything that emerges from his mouth” (Numbers 30:3). “Moses spoke to the heads of the tribes.… If a man takes a vow to the Lord” – that is what is written: “You will take an oath: As the Lord lives, in truth, in justice and in righteousness” (Jeremiah 4:2).
The Holy One blessed be He said to Israel: Do not think that it is permitted for you to take an oath in My name, even truthfully. You are permitted to take an oath in My name only if all these attributes exist in you: “The Lord your God you shall fear […and by his name you shall take an oath]” (Deuteronomy 6:13, 10:20) – that you shall be like those who called God-fearing: Abraham, Job, and Joseph.
Abraham, as it is written: “For now I know that you are God-fearing” (Genesis 22:12). Job, as it is written: “A virtuous and upright man, fearing God” (Job 1:8). Joseph, as it is written: “I fear God” (Genesis 42:18). That is, “the Lord your God you shall fear.”
“Him you shall serve” (Deuteronomy 6:13, 10:20) – if you devote yourself to Torah and to engaging in mitzvot, and you have no other work; that is why it is stated: “Him you shall serve.” “To Him you shall cleave” (Deuteronomy 10:20) – can a person cleave to the Divine Presence? Is it not already stated: “For the Lord your God is a consuming fire” (Deuteronomy 4:24)? Rather, it is to say to you: Anyone who marries his daughter to a scholar who studies Bible and Mishna, and engages in commerce and benefits him [the scholar] from his property, this is the one in whose regard it is stated: “To Him you shall cleave.”
If you have all these qualities you are permitted to take an oath, but if not, you are not permitted to take an oath, There was an incident involving King Yannai, who had two thousand cities and all of them were destroyed due to true oaths. How so? A person would say to another: ‘By oath that I am going and eating this and that in such and such place, and drinking this and that in such and such place.’
They would go and fulfill their oath, and they were destroyed. If for one who takes a true oath it is so, one who takes a false oath, all the more so.