“This is the thing that Hashem has commanded … he shall not desecrate his word, according to whatever comes from his mouth shall he do” (Numbers 30:2-3)
The Abir Yaakov warns us that a person may learn Torah, pray, and fulfill the mitzvot, but if one does not guard oneself in the areas of business dealings or speech, by being honest and acting in good faith, regarding such a person we read, “In vain is the watchman vigilant” (T’hillim 127:1). This person’s Torah is for nothing, G-d forbid.
This is similar to a mincha (grain offering) to Hashem that must be brought in a pure vessel. Even if only the vessel is impure, they are both rejected.
The vessel through which we offer our words of Torah and prayer to Hashem is the mouth. If our mouth becomes contaminated by improper speech, especially by lies and deceit, then any words of Torah and prayer spoken by the same mouth are contaminated and become an abomination to Hashem, G-d forbid. The mouth and the speech we utter are inseparable. If we guard our mouth in mundane matters, then our words of Torah and prayer are acceptable to Hashem. Otherwise, not.
Based on Pituchei Chotam: Insights on the Weekly Parashah by Rabbi Yaakov Abuchatzeira, The Abir Yaakov, Bamidbar — Devarim, Artscrcoll