Nicene and Post-Nicene Fathers, Vol. III:
Moral Treatises of St. Augustin: Section 6

6. On the other hand, those who say that we must never lie, plead much more strongly, using first the Divine authority, because in the very Decalogue it is written “Thou shall not bear false witness;” 2306 under which general term it comprises all lying: for whoso utters any thing bears witness to his own mind. But lest any should contend that not every lie is to be called false witness, what will he say to that which is written, “The mouth that lieth slayeth the soul:” 2307 and lest any should suppose that this may be understood with the exception of some liars, let him read in another place, “Thou wilt destroy all that speak leasing.” 2308 Whence with His own lips the Lord saith, “Let your communication be yea, yea; nay, nay; for whatsoever is more than these cometh of evil.” 2309 Hence the Apostle also in giving precept for the putting off of the old man, under which name all sins are understood, says straightway, “Wherefore putting away lying, speak ye truth.” 2310

Footnotes

Ex. 20.16Exod. xx. 16

Wisdom 1.11. Os quod mentitur. “The mouth that belieth,” E.V., στόμα καταψευδόμενον

Ps. 5.6Psalm v. 6

Matt. 5.37. [See R.V.]

Eph. 4.25Eph. iv. 25