The 216th mitzvah is that we are commanded that a man must marry his brother's wife, if [that brother] died without leaving children.1
The source of this commandment is G‑d's statement2 (exalted be He) "Her yavam [i.e. the surviving brother] must cohabit with her" [i.e. the sister-in-law].
The details of this mitzvah are found in the tractate devoted to this subject, tractate Yevamot.