Mishnah 10 in the fourth chapter of Tractate Bava Metzia. This chapter and the following chapters deal with monetary laws, but this Mishnah digresses from the topic for a single Mishnah: since we were dealing with financial fraud (onaah), the Mishnah points out that there is another type of onaah that has nothing to do with commerce - causing distress to others using words, which is called onaat devarim.
The source of the prohibition in the Torah:
In chapter 25 of the Book of Vayikra, the same chapter where the mitzvah of onaah we mentioned earlier is stated, an additional verse appears dealing with onaah: "Velo tonu ish et amito veyareita me'eloheicha ki ani Hashem Elokeichem" - a person must not oppress or wrong his fellow, his upright Jewish friend, and he must fear Hashem. This additional verse, dealing with a different type of onaah, is understood to refer to onaat devarim: hurting another person and causing mental anguish through speech.
As the Mishnah states: "Keshem she'onaah bemekach umemkar, kach onaah bidevarim" - just as there is a concept of wronging another through financial exploitation and fraud in commercial activities of buying and selling, so too there is wronging another through words.
The Mishnah brings three examples:
"Lo yomar lo: bechamah chafetz zeh, vehu eino rotzeh likach" - a person asking a seller how much an item costs, when he has no intention of buying it at all. A merchant sitting with his merchandise, and finally a customer comes in and asks: "How much is this diamond ring?", becomes filled with anticipation that a sale will finally be made. When it becomes clear to him that the inquirer does not intend to buy, his spirits fall and he becomes sad. This is one explanation of several, and it is sufficient for our purposes.
The very act of asking the question without intending to buy causes distress to the other person, and therefore it is considered onaat devarim. It is understood that this is a relatively minor distress, and it is difficult to determine to what extent this applies nowadays, since often people are not affected by ordinary purchases; but to some extent, it does apply. The general rule is that when dealing with a person who has nothing to gain or lose from the transaction, it certainly does not apply: checking a price on the internet is not a problem at all, nor is a question directed to an employee stocking shelves, who does not receive a commission and is unaffected by whether a sale is made. However, toward a salesperson who earns commissions, this would seemingly apply.
"Im hayah baal teshuvah, lo yomar lo: zechor maaseicha harishonim" - a person who committed improper acts in the past and corrected his evil ways, one is forbidden to remind him of his past, since this reminder causes him to feel bad and sink into emotional distress. This is onaat devarim.
It is worth noting that the term 'baal teshuvah' in common contemporary parlance refers to a person who did not grow up in a religious family and later became religious, and this is not necessarily its meaning in the Mishnah. It could be a person who grew up in a completely religious family, but fell and deteriorated into a life of crime, and later stopped and returned to the good path of the Torah - he too is called a baal teshuvah, and it is forbidden to remind him of his past.
"Im hu ben gerim, lo yomar lo: zechor maaseh avoteicha" - the third example deals not only with the person's own past, but even with the past of his parents. A person whose parents converted him when he was a small child, and he himself grew up as a good and God-fearing child and did no wrong, but his parents did not grow up this way - it is forbidden to say to him, "Remember what your parents did before they became Jewish." Even though he did nothing wrong, the mere reminder of the evil deeds of his parents is enough to cause him a measure of distress, and this too is onaat devarim.
What these three cases have in common: it is forbidden to cause someone distress through questions directed at them or things said to them.
The verse brought at the end of the Mishnah:
"Veger lo soneh velo silchatzenu" - when dealing with a convert, it is forbidden to "wrong" him, meaning to cause him distress, and it is forbidden to "oppress" him. Chazal understand that "soneh" refers to verbal wronging, whereas "silchatzenu" refers to financial pressure, namely monetary fraud.
The verse brought as proof is not strictly necessary in and of itself, but comes to teach that there is an entirely separate prohibition here. Fundamentally, there are two prohibitions of verbal wronging regarding a convert: the first prohibition, which applies to Jews in general, and the second prohibition, which applies specifically to converts. A convert is, of course, a Jew, and therefore one who causes a convert distress with words violates two negative commandments and not just one.
There is an additional, important difference: the prohibition of verbal wronging toward a convert applies to every convert, and once he has converted, he has special protections. In contrast, regarding a regular Jew it says "Velo sonu ish es amiso", and the Gemara expounds: "amiso" - literally, someone who belongs to your people, but the intention of the exposition is one who acts as a member of the people in a proper state, meaning "he does the deeds of your people", one who behaves like a Jew. A person who does not behave like a good Jew - for example, one who publicly desecrates Shabbos, to whom Shabbos means nothing at all; he is a Jew, but he is not a good Jew - he is not protected by the boundaries of the prohibition of verbal wronging. It turns out that the prohibition does not apply to every Jew, but only to upstanding Jews who behave properly from a halachic perspective, and there is a precise definition for this. However, the second prohibition, which protects converts from verbal wronging, applies to every convert, regardless of their current level of mitzvah observance.
In summary: In this Mishnah we learned that alongside monetary fraud there is verbal wronging - causing distress to another through speech, which is derived from the verse "Velo sonu ish es amiso". Three examples were brought: asking for a price with no intention of buying, reminding a baal teshuvah of his past, and reminding the son of converts about the deeds of his ancestors. We also noted that there is an additional, special prohibition regarding a convert derived from "Veger lo soneh velo silchatzenu", and that while the prohibition of verbal wronging for a regular Jew applies specifically to someone who "does the deeds of your people", the protection for the convert applies in every situation.