Deuteronomy 17:6

שנים עדים או שלשה [BY THE MOUTH OF] TWO WITNESSES OR THREE [WITNESSES, SHALL HE THAT IS WORTHY OF DEATH BE PUT TO DEATH] — But if evidence can be established by two witnesses to what end does it (Scripture) mention to you explicitly that it may be established by three? Scripture does so in order to compare the evidence of three witnesses to that of two (to make the same law apply to both cases). How is it in the case of two? Their evidence forms one testimony; so, too, does the evidence of three (or many) witnesses form one testimony and they cannot be declared “plotting witnesses” unless all of them are proved to be “plotting witnesses” (Makkot 5b).

עַל־פִּ֣י ׀ שְׁנַ֣יִם עֵדִ֗ים א֛וֹ שְׁלֹשָׁ֥ה עֵדִ֖ים יוּמַ֣ת הַמֵּ֑ת לֹ֣א יוּמַ֔ת עַל־פִּ֖י עֵ֥ד אֶחָֽד׃

At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; but at the mouth of one witness he shall not be put to death.