TT #230 · 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 Sanhedrin 37b.
TT lemma: לֹא יָקוּם עֵד אֶחָד בְּאִישׁ.
English: ‘A single witness shall not rise up against a man’ – for punishment, but for an oath he does rise.
Derash limiting the exclusion (Bavli) — one witness is powerless for liability but can obligate an oath.
A single witness cannot create liability, yet in monetary matters he can force the defendant to take an oath.