Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 178
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 178, with English translation by Maimonides (Rambam).
להעיד בבית דין למי שיש לו עדות, שנאמר: "והוא עד, או ראה או ידע" (ויקרא ה,א).
For anyone who has evidence to testify in court, as [Leviticus 5:1] states: "If he was a witness, saw, or knew...."