"Thou shalt not bear false witness against thy neighbor." - Exodus 20:16
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From the Westminster Shorter Catechism:
Q. 76. Which is the ninth commandment?
A. The ninth commandment is, Thou shalt not bear false witness against thy neighbour.
Q. 77. What is required in the ninth commandment?
A. The ninth commandment requireth the maintaining and promoting of truth between man and man, and of our own and our neighbour’s good name, especially in witness-bearing.
Recommended Sermon: WSC #73-#75: Thou Shalt Not Steal, by Pastor David Reese [Text: Deuteronomy 23:15 (Audio); Exodus 20:15 (Audio)]