Numbers 5:19

והשביע אתה וגו׳ AND [THE PRIEST] SHALL ADJURE HER etc. — and wherein does this adjuration consist? In that he says to the woman, “if no man hath lain with thee … הנקי, be thou free”, which implies: “if, however, a man hath lain with thee, חנקי, thou deservest to be suffocated”, because from a negative statement you may deduce its converse, the positive statement. It is true that in an adjuration it would be more appropriate to make this threat of punishment if guilty, and to allow the converse to be derived from it, but here it mentions first what will happen if she has not sinned, since it intends to suggest that it is a duty in capital cases to begin the proceedings with the statement of something that bears upon the innocence of the accused (Sotah 17a; Shevuot 36a; Sanhedrin 33a).

וְהִשְׁבִּ֨יעַ אֹתָ֜הּ הַכֹּהֵ֗ן וְאָמַ֤ר אֶל־הָֽאִשָּׁה֙ אִם־לֹ֨א שָׁכַ֥ב אִישׁ֙ אֹתָ֔ךְ וְאִם־לֹ֥א שָׂטִ֛ית טֻמְאָ֖ה תַּ֣חַת אִישֵׁ֑ךְ הִנָּקִ֕י מִמֵּ֛י הַמָּרִ֥ים הַֽמְאָרְרִ֖ים הָאֵֽלֶּה׃

and the priest shall charge her by an oath, and say to the woman, If no man have lain with thee, and if thou hast not gone aside to uncleanness with another instead of thy husband, be thou free from this bitter water that causes the curse: