אֽוֹ־מָצָ֧א אֲבֵדָ֛ה וְכִ֥חֶשׁ בָּ֖הּ וְנִשְׁבַּ֣ע עַל־שָׁ֑קֶר עַל־אַחַ֗ת מִכֹּ֛ל אֲשֶׁר־יַעֲשֶׂ֥ה הָאָדָ֖ם לַחֲטֹ֥א בָהֵֽנָּה׃
O sea que hallando lo perdido, después lo negare, y jurare en falso, en alguna de todas aquellas cosas en que suele pecar el hombre:
Rashi
ולא תכחשו NEITHER SHALL YE DEAL FALSELY [WITH ONE ANOTHER] — Since Scripture has stated (Leviticus 5:21, 22) "[If a man sin… and deny unto his neighbour a charge, or a deposit… or has found that which was lost] and denieth it (וכחש בה)", that he shall pay the principal and add a fifth part more thereto, we have there mention only of the punishment he incurs; whence do we derive the prohibition (i.e. where is it forbidden)? From Scripture's statement here “neither shall ye deal falsely" (The Hebrew here is תכחשו, the same term as is used in the verse quoted — וכחש בה) (Sifra, Kedoshim, Section 2 3; Bava Kamma 105b).
Rashbam
לא תכחשו, denial that one had received a deposit for safekeeping from another Jew. Compare Leviticus 5,22.
Chizkuni
לא תכחשו ולא תשקרו, “do not deal falsely nor lie to one another.” The prohibition is spelled out here; where do we have the penalty for violating this commandment? According to the (Sifra) we find it in Leviticus 5,22 in the words: וכחש בה וחמשיתו יוסף עליו, “if he denies it he will have to add a fifth of its value when making good.”After G-d had commanded us to give some of what we own to Him, he also warns us not to deprive our fellowman of what is rightfully his. (Ibn Ezra, worded slightly differently)