Berachot 17a — Guard my tongue from evil and my lips from speaking falsehood.
Rambam, Hilchot De’ot 2:6 — It is forbidden to accustom oneself to smooth speech and flatteries. One must not say one thing and mean another. Inward and outward self should correspond; only what we have in mind should we utter with the mouth. We must deceive no one, not even an idolater... even a single word of flattery or deception is forbidden. A person should always cherish truthful speech, an upright spirit, and a pure heart freed of all pretense and cunning.
Shmot 20:12 — You shall not murder, you shall not commit adultery, you shall not steal, you shall not bear false witness against your fellow.
Vayikra 19:11 — You shall not steal, neither shall you deal falsely nor lie one to another.
