Augustine of Hippo
Catena Aurea · Matthäusevangelium 5
Again, ye have heard that it hath been said by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths: But I say unto you, Swear not at all; neither by Heaven; for it is God’s throne; Nor by the earth; for it is His footstool: neither by Jerusalem; for it is the city of the great King. Neither shalt thou swear by thy head, because thou canst not make one hair white or black. But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
(ubi sup.) And he who has learned that an oath is to be reckoned not among things good, but among things necessary, will restrain himself as much as he may, not to use an oath without necessity,unless he sees men loth to believe what it is for their good they should believe, without the confirmation of an oath. This then is good and to be desired, that our conversation be only, yea, yea; nay, nay; for what is more than this cometh of evil. That is, if you are compelled to swear, you know that it is by the necessity of their weakness to whom you would persuade any thing; which weakness is surely an evil. What is more than this is thus evil; not that you do evil in this just use of an oath to persuade another to something beneficial for him; but it is an evil in him whose weakness thus obliges you to use an oath.