In the previous mitzvah, we said that if one has another person’s property in his possession, it is sinful to deny it. Here, we kick things up a notch. If the one holding on to his friend’s property falsely denies possession with an oath (“I swear I don’t have it!”), it constitutes a completely separate violation. (According to the Talmud in tractate Shevuos, page 49b, swearing falsely about possession of another’s property actually violates both this mitzvah and the next.)
The reason for this mitzvah is the importance of truth. As we discussed in Mitzvah #37, the prohibition against perjury, the seal of God is truth (Sanhedrin 64a) and He tells us in the Torah to distance ourselves from every false matter (Exodus 23:7). Truth is an important foundation of society and if oaths are used falsely, we’re in really bad shape because we just can’t trust anything.
This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in the fifth of tractate Shevuos, starting on page 36b, and elsewhere around the tractate. It is codified in the Shulchan Aruch in Choshen Mishpat 367. This prohibition is #249 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #30 of the 194 negative mitzvos that can be fulfilled today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.