Devarim 25:9

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

וְנִגְּשָׁ֨ה יְבִמְתּ֣וֹ אֵלָיו֮ לְעֵינֵ֣י הַזְּקֵנִים֒ וְחָלְצָ֤ה נַעֲלוֹ֙ מֵעַ֣ל רַגְל֔וֹ וְיָרְקָ֖ה בְּפָנָ֑יו וְעָֽנְתָה֙ וְאָ֣מְרָ֔ה כָּ֚כָה יֵעָשֶׂ֣ה לָאִ֔ישׁ אֲשֶׁ֥ר לֹא־יִבְנֶ֖ה אֶת־בֵּ֥ית אָחִֽיו׃

then shall his brother’s wife approach him in the presence of the elders, and loose his shoe from off his foot, and spit in his face, and shall answer and say, Thus shall it be done to that man that will not build up his brother’s house.