Sefer HaMitzvot, Positive Commandments
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היא שצונו להיות היבם נושא את אשת אחיו כשמת ולא הניח זרע. והוא אמרו יתעלה יבמה יבא עליה. וכבר התבארו דיני מצוה זו במסכת יבמות. (כי תצא, נשים הלכות יבום וחליצה):
Sefaria Edition 2021, Translated by Rabbi Francis Nataf
That is that He commanded us that a yavam (levirate husband) marry the wife of his brother when [the latter] dies without leaving offspring. And that is His, may He be exalted, saying, "her yavam will have intercourse with her" (Deuteronomy 25:5). And the laws of this commandment have already been explained in Tractate Yevamot. (See Parashat Ki Tetzei; Mishneh Torah, Levirate Marriage and Release.)