Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 205
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 205, with English translation by Maimonides (Rambam).
להוכיח החוטא, שנאמר: "הוכח תוכיח את עמיתך" (ויקרא יט,יז).
To rebuke a person who sins, as [Leviticus 19:17] states: "You shall surely rebuke your fellow men."