Rashi on Deuteronomy 25:9/רש"י על דברים כ״ה:ט׳

1וירקה בפניו THEN SHE SHALL SPIT BEFORE HIS FACE — on the ground (not in his face) (Yevamot 106b).

2אשר לא יבנה [SO SHALL IT BE DONE UNTO THAT MAN] THAT WILL NOT BUILD UP [HIS BROTHER’S HOUSE] — from here the Rabbis derived the law that one who has once given his deceased brothers wife חליצה shall not afterwards marry her, for it does not state “[Thus shall it be done to the man] who hath not built up his brother’s house”, but אשר לא יבנה: “[Thus shall be done to the man] that he shall never build up” — because he has not built up, he shall never after build it up (Yevamot 10b; cf. Sifrei Devarim 291:8).