(Numbers 5:13:) AND THERE IS NO WITNESS AGAINST HER. Even though she has none now, she has one after a time. In a similar matter you say (Genesis 11:30): AND SARAI WAS BARREN; SHE HAD NO CHILD. Even though she had none then, she had one after a time, as it is said (Genesis 21:1): AND THE LORD VISITED SARAH. In a similar matter you say (Esther 2:10): ESTHER HAD NOT TOLD. Even though she had not told him then, she told him after a time, as it is said (Esther 8:1): AND MORDECAI CAME BEFORE THE KING, FOR ESTHER HAD TOLD WHAT HE WAS TO HER. And here too (Numbers 5:13): AND THERE IS NO WITNESS AGAINST HER. Even though she has none now, she has one after a time, as it is said (Malachi 3:5): AND I WILL BE A SWIFT WITNESS [AGAINST THE SORCERERS AND AGAINST THE ADULTERERS].
No Witness Against Her Now but God Will Witness Later
Midrash Tanchuma Buber, Nasso 12:1
ועד אין בה (במדבר ה יג). אע"פ שאין לה עכשיו, יש לה לאחר זמן, כיוצא בדבר אתה אומר ותהי שרי עקרה אין לה ולד (בראשית יא ל), אע"פ שאין לה עכשיו, יש לה לאחר זמן, שנאמר וה' פקד את שרה וגו' (שם כא א), כיוצא בדבר אתה אומר, לא הגידה אסתר (אסתר ב י), אעפ"כ שלא הגידה לו עכשיו, הגידה לו לאחר זמן, שנאמר ומרדכי בא לפני המלך כי הגידה אסתר מה הוא לה (שם ח א), ואף כאן ועד אין בה, אע"פ שאין לה עכשיו, יש לאחר זמן, שנאמר והייתי עד ממהר [במכשפים ובמנאפים] (מלאכי ג ה).