신명기 25:6의 미드라쉬

(Devarim 25:6) "And it shall be, the first-born that she bears shall be invested in the name of his dead brother": I might think that if his (the dead brother's) name were Yossi, he (the child) should be called Yossi, or (if his name were) Yochanan, the child should be called Yochanan; it is, therefore, written "in the name of his (the yavam's) dead brother" and not "in the name of the dead brother of his (the child's) father." [(The "first-born," then, in this instance, is an epithet for the yavam.)] If so, why is it written "the first-born" (instead of "the yavam")? To indicate that it is a mitzvah for the oldest brother to perform the yibum.