Positive commandment · Mitzvat Aseh #205 of 613
To rebuke a person who sins
להוכיח החוטא
The verse · Vayikra 19:17
לֹא־תִשְׂנָא אֶת־אָחִיךָ בִּלְבָבֶךָ הוֹכֵחַ תּוֹכִיחַ אֶת־עֲמִיתֶךָ וְלֹא־תִשָּׂא עָלָיו חֵטְא
“Thou shalt not hate thy brother in thy heart: thou shalt certainly rebuke thy neighbour, and not suffer sin on his account.”
The Rambam’s words
To rebuke a person who sins, as [Leviticus 19:17] states: "You shall surely rebuke your fellow men."
להוכיח החוטא, שנאמר: "הוכח תוכיח את עמיתך" (ויקרא יט,יז).
Why · Sefer HaChinuch #239
Rebuke to an Israelite who does not behave properly
“It is from the roots of the commandment [that it is] because there is peace and goodness between people with this. As when a man sins to a man, and he rebukes him privately, he will apologize in front of him, and [the other] will accept his apology and he will be whole (some have the variant, at peace) with him.”
Who & when
“And this commandment is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Human Dispositions הלכות דעות