Gn38-08a, presenting Judah's request to Onan to marry his sister-in-law after his brother, Er, died, states And Judah said to Onan, Go in to your brother’s wife, and yabem her, and raise up seed to your brother. Rashi clarifies the meaning of the underlined Hebrew word yabem by citing an other verse where this word is defined: Dt25-05:06 discussing the childless widow states If brothers live together, and one of them dies, and has no child, the wife of the dead shall not marry outside to a stranger; her husband’s brother shall yabem her, and take her to him for a wife, and perform the duty of a husband’s brother to her. And it shall be, that the firstborn which she bears shall succeed to the name of his brother who is dead, that his name be not put out of Israel. Using this other verse Rashi explains that the Hebrew term yabem refers to a marriage of a childless widow by a surviving brother-in-law in order to perpetuate the name of the deceased. Rashi concludes that a similar usage exists in Gn38-08a.

Advanced Rashi: Here the Bible uses a term, yibum that predates the actual creation of this procedure. Such foreshadowing is common in the Bible. For example Gn02-01:04 mentions that God sanctified the Sabbath, referring to the future Jewish custom of abstaining from work on the Sabbath and making the day a holy day. Such foreshadowing linguistically encourages respect for Biblical rituals.