(13) You shall not murder. You shall not commit adultery. You shall not steal. You shall not bear false witness against your neighbor.
(2) לא תגנוב, the term “stealing” also includes the “stealing” of human beings i.e. kidnapping. Even deceiving your fellow man deliberately is called “stealing” גנבת דעת הבריות, “stealing people’s minds", misleading them to believe that lies are truth. (compare Sanhedrin 86)
(ג) שבעה גנבין הן: הראשון שבכולן גונב דעת הבריות והמסרב בחבירו לאורחו ואין בלבו לקרותו והמרבה לו בתקרובות ויודע בו שאינו מקבל.
There are seven types of thieves, the first is one steals knowledge, such as someone who is invites guests over knowing that this person will not accept the invitation or someone who sends gifts to someone knowing that a person will send them back.
(ו) אסור לאדם להנהיג עצמו בדברי חלקות ופיתוי ולא יהיה אחד בפה ואחד בלב אלא תוכו כברו והענין שבלב הוא הדבר שבפה ואסור לגנוב דעת הבריות ואפילו דעת הנכרי כיצד לא ימכור לנכרי בשר נבילה במקום בשר שחוטה...ולא יסרהב בחבירו שיאכל אצלו והוא יודע שאינו אוכל ולא ירבה לו בתקרובת והוא יודע שאינו מקבל ולא יפתח לו חביות שהוא צריך לפותחן למוכרן כדי לפתותו שבשביל כבודו פתח וכן כל כיוצא בו ואפילו מלה אחת של פיתוי ושל גניבת דעת אסור אלא שפת אמת ורוח נכון ולב טהור מכל עמל והוות.
A person is forbidden to act in a deceptive and smooth-tongued manner. He should not speak one thing outwardly and think otherwise in his heart. Rather, his inner self should be like the self which he shows to the world. What he feels in his heart should be the same as the words on his lips. It is forbidden to deceive people, even a non-Jew. For example, one should not sell a gentile the meat of an animal which has not been ritually slaughtered as if it were ritually slaughtered meat...One should not press his colleague to share a meal with him when he knows that his colleague will not accept the invitation, nor should he press presents upon him when he knows that his colleague will not accept them. He should not open casks supposedly for his colleague which he must open for sale, in order to deceive him into thinking that they have been opened in his honor. The same applies with all matters of this sort.
It is forbidden to utter a single word of deception or fraud. Rather. one should have only truthful speech, a proper spirit and a heart pure from all deceit and trickery.
Mar Zutra (a great rabbi) son of R. Nahman was going from the town of Sichra to Mehuzah, while Rava and R. Safra were also heading towards Sichra. They met on the road, and Mar Zutra thought they came out to greet him. He said, "Why would you make such a great effort for this?" R. Safra responded, "We didn't know you were coming, but if we did, we would have made an even greater effort." Rava said later on to R. Safra "Why did you say that to him. you made him feel bad!" Rav Safra responded "It would have been deceiving him if we didn't say anything!".
(ו) ואם הוא דבר דאי בעי ליה לאסוקי אדעתיה שאינו עושה בשבילו ומטעה עצמו שסובר שעושה בשבילו לכבודו כגון שפגע בחבירו בדרך וסבור זה שיצא לקראתו לכבדו אין צריך להודיעו:
If the recipient should have realized that nothing was done for him, but he is mistaken and thinks that this was done for his honor such as when a friends meets someone on the street, and the friend thought that the person had come specifically to greet him, it is not necessary to inform the friend otherwise.
בני, בנימין, הלכות דעות ב:ו
ומשמע שם שאפילו אינו בדבר שבממון אסור שאילו בממון אסור משום לא תגנוב שהיא גניבת דעת.
Bene Binyamin, Hilchot De'ot 2:6
[About the Rambam’s ruling] implicit from that passage that it is prohibited even if there are no monetary consequences, for if there were monetary consequences, it would be a violation of “do not steal” which includes geneivat da’at.