Reference for Pirkei Avot 1:9

שִׁמְעוֹן בֶּן שָׁטָח אוֹמֵר, הֱוֵי מַרְבֶּה לַחְקֹר אֶת הָעֵדִים, וֶהֱוֵי זָהִיר בִּדְבָרֶיךָ, שֶׁמָּא מִתּוֹכָם יִלְמְדוּ לְשַׁקֵּר:

Shimon ben Shetach says: Be thorough in cross-examining the witnesses, and be circumspect in your words, lest from them they learn to lie. [Let the judge not say: "Perhaps this and this is what happened," or "If this were the case, Ploni would be innocent." From such words, the litigant or the witnesses learn to say what never happened.]

Jastrow