Tarry this night, and it shall be in the morning, that if he will perform unto thee the part of a kinsman, well; let him do the kinsman's part: but if he will not do the part of a kinsman to thee, then will I do the part of a kinsman to thee, as the LORD liveth: lie down until the morning. As the Lord liveth - Thus he bound himself by an oath to take her to wife if the other should refuse.
Related Commentaries of Ruth 3:13
Ruth 3:13
13 Tarry this night, and it shall be in the morning, that if he will perform unto thee the part of a kinsman, well; let him do the kinsman's part: but if he will not do the part of a kinsman to thee, then will I do the part of a kinsman to thee, as the LORD liveth: lie down until the morning.