Мелахим А 1:21 Галаха: Mishneh Torah, Human Dispositions

He who beholds his fellow stooping to sin or following an unrighteous path, is obliged to return him toward the good, and to let him know that he is actually sinning against himself in pursuing wicked deeds for, it is said: "And thou shalt indeed rebuke thy neighbor" (Lev. 19.17). He who rebukes his fellow, whether it be regarding a sin committed between man and man, or whether it be regarding matters between man and God, it is essential that the rebuke be administered only between them both; and he shall speak to him calmly, employing soft language, telling him that he does not speak of it to him, save for his own good, to bring him to a life in the world to come. If he receive it attentively from him, it is well; if not, he should rebuke him a second, even a third time. So is the constant duty of a man to continue to rebuke his fellow, even until the sinner strike him, and say unto him: "I will not listen". He in whose power it is to prevent sin and does not take the means to prevent it, he himself is ultimately overtaken by their sin, since it was possible for him to prevent them.⁷Shabbat, 54a; Ibid. 119b; Baba Mezi’a 31a. C. G.