The 149th mitzvah is that we are commanded regarding the signs of beheimos and chayos [animals1]. They must chew their cud and have split hooves, and only then may they be eaten. It is a positive commandment that we examine them for these signs.

The source of this commandment is G‑d's statement,2 "These are the animals that you may eat...."

The Sifra says, "The phrase,3 '[Among mammals, any one that has split hooves that are cloven and that brings up its cud —] that one you may eat,' teaches that only that kind may you eat, and you may not eat one which is non-kosher." This means that from the law that it may be eaten if it has these signs, we imply that it may not be eaten if it lacks these signs. And according to the principle already explained,4 a prohibition which is implied from a positive commandment is counted as a positive commandment. Therefore, after the passage quoted above, the Sifra continues, "This teaches us the positive commandment; what is the source of the prohibition? The verse,5 ['these are the ones that you may not eat...:] The camel...' " as explained in the section dealing with the prohibitions.6

This shows that the statement, "That one you may eat," constitutes a positive commandment. The mitzvah, as previously mentioned, is that we are commanded to inspect every beheimah and chaya for these signs; and only then may it be eaten. This law is itself the mitzvah.

The details of this mitzvah are explained in tractates Bechoros and Chullin.