Halakhah for Shevuot 70:7

To swear in His name, may He be blessed, truthfully: To swear in His name, blessed be He, at the time that we need to strengthen or establish a thing or to distance it; as with this is there aggrandizement of His statute, may He be blessed, and [of] the Power and the Loftiness. And about this is it stated (Deuteronomy 10:20) "and in His name shall you swear." And in the explanation, they, may their memory be blessed, said (Shevuot 35b), "The Torah stated, 'Swear in His name,' and the Torah said, 'Do not swear in His name'" - meaning to say, [just] like an oath that is not needed [should] be prevented, and that is a negative commandment; so too is an oath at a time that it is needed an obligation, and that is a positive commandment. And therefore we do not ever swear in [the name of] any thing of all the creatures. And they, may their memory be blessed, said (Sanhedrin 63a), "Anyone who combines the name of [God] with something else is uprooted from the world." However this is said about one who intends to swear in that thing of the creatures by itself; but one who swears in the heavens or in the sun or in the moon, with the intention of the Master above them that created them - that is not included in the prohibition at all. And we always see that we swear like this in all of the borders of Israel.