The 222nd mitzvah is that when we wish to divorce, we are commanded to do so only with a written document [i.e. a get].

The source of this commandment is G‑d's statement1 (exalted be He), [that when a man wants to divorce his wife,] "he shall write her a bill of divorce and place it in her hand."

The details of this mitzvah, i.e. all the laws of divorce, are found in the tractate devoted to this subject, tractate Gittin.