TT #235 · 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: לֹא יָקוּם עֵד אֶחָד.
English: ‘A single witness shall not rise up’ – to punish him, but to treat him more stringently in the hands of Heaven he does rise.
Derash distinguishing human and heavenly judgment (Sifrei) — one witness is powerless in court but not before Heaven.
A single witness cannot bring about a human court’s penalty, yet the matter may still stand before Heaven.