Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 288
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 288, with English translation by Maimonides (Rambam).
שלא לכרות הדין על פי עד אחד, שנאמר "לֹא יָקוּם עֵד אֶחָד בְּאִישׁ" (דברים יט, טו).
Not to render a decision based on the testimony of a single witness, as [Deuteronomy 19:15] states: "A single witness shall not rise up against a person."