Deuteronomy 19:18

ודרשו השפטים היטב AND THE JUDGES SHALL DILIGENTLY ENQUIRE concerning the statement of those who assert them (the first witnesses) to be “plotting witnesses”, in that they investigate and crossexamine those who assert them to be “plotting witnesses” by diligent enquiry and scrutiny. והנה עד שקר העד AND, BEHOLD, IF THE WITNESS BE A FALSE WITNESS — Wherever עד is written, [except if the numeral אחד, “one” is added (cf. Rashi on v. 15)], Scripture is speaking of two witnesses (Sanhedrin 30a).

וְדָרְשׁ֥וּ הַשֹּׁפְטִ֖ים הֵיטֵ֑ב וְהִנֵּ֤ה עֵֽד־שֶׁ֙קֶר֙ הָעֵ֔ד שֶׁ֖קֶר עָנָ֥ה בְאָחִֽיו׃

and the judges shall make diligent inquiry: and, behold, if the witness is a false witness, and has testified falsely against his brother;