LXX Deut 19:15

Οὐκ ἐμμενεῖ μάρτυς εἷς μαρτυρῆσαι κατὰ ἀνθρώπου κατὰ πᾶσαν ἀδικίαν καὶ κατὰ πᾶν ἁμάρτημα καὶ κατὰ πᾶσαν ἁμαρτίαν ἣν ἂν ἁμάρτῃ· ἐπὶ στόματος δύο μαρτύρων καὶ ἐπὶ στόματος τριῶν μαρτύρων στήσεται πᾶν ῥῆμα.

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.