Musar על יבמות 130:6

For instance, the Torah commanded us: "you shall surely rebuke your fellow" (Vayikra 19:17). Very often a person attempts to rebuke sinners at a place or time when his words will not be heeded and he causes them to breach even further in their wickedness, to desecrate the Name of G-d, and to add transgression to their sin. In such cases, the only Chasidut is to keep silent. Thus, our sages, of blessed memory, said: "just like it is a mitzva to say what will be heeded, so too it is a mitzva to not say what will not be heeded" (Yevamot 65b).

And they said: Just as it is a commandment to say a thing that will be heard, so is it a commandment not to say a thing that will not be heard (Yebamoth 65b). And our Sages said, "Better that they (the sinners) should commit a wrong not knowing that it is a wrong, than that they should commit that wrong intentionally" (Shabbath 148b).