Refuse altogether to take an oath if you can, if not, as far as may be.
(CLXVI, The Golden Sayings of Epictetus, trans. by Hastings Crossley, 1909)
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.