John Gill Commentary Matthew 5:33

"Again, ye have heard that it was said to them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths:" — Matthew 5:33 (ASV)

Again, you have heard that it has been said
Besides what has been observed, in ver. 21 and 27 you know it has also been said,

by, or to them of old time ,
what is written in (Leviticus 19:12) . "And you shall not swear by my name falsely"; which seems to be referred to, when it is said, "you shall not forswear yourself": and is the law forbidding perjury, or false swearing; and was what the Jews were chiefly, if not only concerned about; little regarding the vanity, only the truth of an oath.

For they took swearing vainly, to be the same as swearing falsely; wherefore so long as what they swore was truth, they were not careful whether it was of any importance or not: moreover, these men sinned, in that they swore by the creatures, which they thought they might do, and not sin; and when they had so done, were not under obligation to perform; because they made no use of the name of God, to whom only vows and oaths were to be performed.

"but shall perform to the Lord your oaths", (Numbers 30:2) which they understood of vows only made to the Lord, and not to others; and of oaths, when in his name, and not by others; which they did do, and yet thought themselves not obliged by them.