28 It is also written, “But I say unto you, Swear not at all.” But the Apostle himself has used oaths in his Epistles. And so he shows how that is to be taken which is said, “I say unto you, Swear not at all:” that is, lest by swearing one come to a facility in swearing, from facility to a custom, and so from a custom there be a downfall into perjury. And therefore he is not found to have sworn except in writing, where there is more wary forethought, and no precipitate tongue withal.
And this indeed came of evil, as it is said, “Whatever is more than these is of evil:” not however from evil of his own, but from the evil of infirmity which was in them, in whom he even in this way endeavored to work faith. For that he used an oath in speaking, while not writing, I know not that any Scripture has related concerning him. And yet the Lord says, “Swear not at all:” for He has not granted license thereof to persons writing. Howbeit, because to pronounce Paul guilty of violating the commandment, especially in Epistles written and sent forth for the spiritual life and salvation of the nations, were an impiety, we must understand that word which is set down, “At all,” to be set down for this purpose, that as much as in you lies, you affect not, love not, nor as though it were for a good thing, with any delight desire, an oath.
Source: On Lying (New Advent)