The 113th mitzvah is that we are commanded to prepare the red heifer1 to have it ready for the procedure of purifying those who are tameh as a result of tumas meis.2

The source of this commandment is G‑d's statement3 (exalted be He), "[A ritually clean person shall gather up the cow's ashes and...] it shall be a keepsake for the congregation of the Jewish people."

The details of this mitzvah are found in the tractate devoted to this subject, tractate Parah.