TT #227 · Parashas Shoftim · Sixth Aliyah · Passuk 19:15
A single witness shall not rise up against a man for any iniquity or for any sin, in any sin that he may commit; by the mouth of two witnesses or by the mouth of three witnesses a matter shall be established.
Torah Temimah points to Sifrei Devarim 188.
TT lemma: עַל פִּי שְׁנֵי עֵדִים.
English: ‘By the mouth of two witnesses’ – and not by writing alone.
Derash on the mode of testimony (Sifrei) — the phrase ‘by the mouth’ emphasizes spoken testimony.
Judicial establishment of a matter requires the oral testimony of two witnesses, not merely written documents.