Q: 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.

Zechariah 8:16. These are the things that ye shall do; Speak ye every man the truth to his neighbour; execute the judgment of truth and peace in your gates.
1 Peter 3:16. Having a good conscience; that, whereas they speak evil of you, as of evildoers, they may be ashamed that falsely accuse your good conversation in Christ.
Acts 25:10. Then said Paul, I stand at Caesar's judgment seat, where I ought to be judged: to the Jews have I done no wrong, as thou very well knowest.
3 John 12. Demetrius hath good report of all men, and of the truth itself: yea, and we alsobear record; and ye know that our record is true.
Proverbs 14:5. A faithful witness will not lie: but a false witness will utter lies.
Proverbs 14:25. A true witness delivereth souls: but a deceitful witness speaketh lies.

Westminster Project - Francis Beattie on the Second Table of the Law as a means of grace