הלכה על תמורה 6:17

And it is practiced in every place and at all times by males and females. And one who transgresses this and does not want to swear in His name at a time it is needed has violated this positive commandment, according to Rambam, may his memory be blessed. But Ramban, may his memory be blessed, wrote (on Mitzovt Ase 7; Ramban on Deuteronomy 6:13) that an oath in His name [even] at a time of need is not a positive commandment at all; that if we want, we swear, and if we do not want to ever swear, there is no [problem] with this. And there is also a commandment in the prevention from an oath, like the matter that they said in Midrash Tanchuma, Matot 1, "The Holy One, blessed be He, said to them, 'Do not reason that it is permitted to you to swear in My name even truthfully unless there is all of these characteristics with you: "The Lord, your God, you shall fear, and to Him shall you cling." And afterwards, "and in His name shall you swear."'" And if we want, we can say that "and in His name shall you swear" comes to give a positive commandment [alongside the] negative commandment on the one who swears in the name of idolatry; meaning to say, in His name should he swear and not in the name of other gods. And the teacher (Ramban), may his memory be blessed, already wrote about the matter that they, may their memory be blessed, said (Temurah 3b) that we swear to perform a commandment, that we derive it from "to Him shall you cling."