Masechet Bechorot, Perek Dalet, Mishnah Tet. Mishnah says that if somebody is suspected of selling terumah as Chulin (regular), he has terumah produce and he sold it as regular Chulin. Terumah can only be eaten by Kohanim (priests); it's the portion separated from produce grown in Eretz Israel for the Kohanim. There's a limited market for terumah; it can only be purchased and eaten by Kohanim who are pure. So what does the person do over here? He claims that it's regular Chulin. Regular Chulin could be eaten by anybody, so he's trying to increase his customer base, and he's lying. But he's in this situation causing buyers who are not Kohanim or Kohanim who are not Tahor (pure) to make an aveirah, a sin, when they eat this terumah.
So, we say we don't do business with this person; we don't buy from him even water or salt. Water and salt are not bound by the laws of terumah, but still, as a penalty, we don't do business with this individual. We forbid people from buying any food from him, even those foods that are not connected to the laws of terumah. This is the opinion of Rabbi Yehuda. Rabbi Shimon says that whatever has a connection to terumah, to ma'asrot (tithes), we don't take from him. Only foods with terumah and ma'aser that have to be separated are not allowed to be bought from him, but other foods like water and salt, which we mentioned in the beginning of the Mishnah, are allowed to be purchased from him.