TT #225 · 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 / Sanhedrin.
TT lemma: לֹא יָקוּם עֵד אֶחָד.
Sefaria — Sifrei Devarim 188 / Sanhedrin
English: ‘A single witness shall not rise up against a man’ – a prohibition against accepting the testimony of a single witness.
Derash establishing the two-witness rule (Sifrei / Bavli) — one witness is insufficient for any penal consequence.
No judicial penalty or liability may be imposed on the basis of a single witness. Two or three witnesses are required.