The Torah's foundational prohibition is stated plainly in Vayikra 19:11: "You shall not steal; you shall not deal deceitfully or falsely with one another," establishing that creating a false impression in another's mind is biblically prohibited.
The Gemara in Chullin 94a records Shmuel's ruling that it is forbidden to deceive people — and even a gentile — by allowing them to form a mistaken impression of one's effort or generosity, illustrating that genevat da'at applies even when no explicit lie is spoken.
The Rambam (Mishneh Torah, Human Dispositions 2:6) codifies that it is forbidden to steal the minds of people, adding that even a single word of enticement or deception is prohibited, and that one must instead maintain truthful speech, an upright spirit, and a pure heart.
The Rambam (Mishneh Torah, Sales 18:1) further rules that it is forbidden to deceive people in a transaction or to beguile them, and that even deceiving people with words alone — without any monetary transaction — is prohibited.
The Sha'ar HaTeshuvah (Sha'arei Teshuvah 3:184) records that the Sages declared genevat da'at to be considered more severe than outright robbery, precisely because it involves the guilt of false speech.
The Mesillat Yesharim (Mesillat Yesharim 11:5) extends this principle by ruling that it is not only the explicit act of theft or oppression that is forbidden, but anything that conceals a defect or leads another to a false understanding, and he establishes this as a general principle of faithful dealing.
מכתב מאליהו, חלק א עמוד עמוד רצט describes the inner dynamic of self-deception as a form of "genevat da'at elyonah" — a theft of the supernal mind — framing the mixing of truth and falsehood as a fundamental moral failure rooted in a willingness to seat falsehood in the place of truth.