Deut 19:15

לֹֽא־יָקוּם֩ עֵ֨ד אֶחָ֜ד בְּאִ֗ישׁ לְכָל־עָוֺן֙ וּלְכָל־חַטָּ֔את בְּכָל־חֵ֖טְא אֲשֶׁ֣ר יֶֽחֱטָ֑א עַל־פִּ֣י שְׁנֵ֣י עֵדִ֗ים א֛וֹ עַל־פִּ֥י שְׁלֹשָֽׁה־עֵדִ֖ים יָק֥וּם דָּבָֽר׃

A lone witness is not sufficient to establish any wrongdoing or sin against a man, regardless of what offense he may have committed. A matter must be established by the testimony of two or three witnesses.