Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 179
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 179, with English translation by Maimonides (Rambam).
לחקור העדים הרבה, שנאמר: "ודרשת וחקרת ושאלת, היטב" (דברים יג,טו).
To cross-examine the witnesses thoroughly, as [Deuteronomy 13:15] states: "You must investigate and probe, making careful enquiry."