The 243rd mitzvah is that we are commanded regarding a paid watchman and a renter, since both are governed by the same law, as our Sages explained,1 regarding the three laws governing the four types of watchmen.2

The source of this commandment is G‑d's statement3 (exalted be He), "If one person gives another a donkey, an ox, a sheep [or any other animal to watch...]"

The details of this mitzvah are explained in the 6th and 9th chapters of tractate Bava Kama, the 3rd and 6th chapters of Bava Metzia, and the 8th chapter of Shavuos.