…and you shall not oppress (Exodus 22:20)
This mitzvah is in the same verse as the previous one. The pasuk tells us not two wrong a convert in two different ways. The Midrash explains that the former means verbally while the latter refers to financial matters. Of course, we are forbidden to cheat or defraud anyone as we will see IY”H in Mitzvah #337 in parshas Behar, but we are specially commanded about a convert. (The result would be that cheating a convert violates two negative mitzvos!)
The verse also gives a logical reason not to wrong converts. The word for a convert is “ger,” literally “stranger.” The Torah tells us that we should know enough to be sensitive to the feelings of a convert because we know first-hand what it’s like to be strangers. Or, as the Talmud says (Baba Metzia 59b), one shouldn’t criticize others for defects he himself possesses. (This is not to say that being a convert is a defect. It’s just an expression like “the pot calling the kettle black.” The meaning is “be nice to newcomers and outsiders because you were a newcomer and an outsider once, yourself!”)
This prohibition applies to both men and women, in all times and places. This mitzvah is discussed in the Talmud in Baba Metzia 59b. It is codified in the Shulchan Aruch in Choshen Mishpat 228. It is #253 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #50 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.