Parashas Masei · Passuk 35:30 · TT note 4
Chabad.org — English translation + Rashi
Torah Temimah points to Sanhedrin 34a (TT note on “לֹא יַעֲנֶה בְנֶפֶשׁ”).
E-Daf — Sanhedrin 34a · Sefaria
English: It was taught: If one of the witnesses said “I have something to say in his favor,” they do not listen to him, as it says “a single witness shall not answer concerning a person.”
A witness may not switch roles to argue for the defendant’s acquittal.
Once a person has testified as a witness in a capital case, he is barred from offering arguments for the defendant’s innocence.