Shemirat HaLashon שמירת הלשון

And when we reflect upon this in truth, we find that the fulfillment of the mitzvah of judging one's friend in the scales of merit and the trait of guarding one's tongue depend on the fulfillment of the positive commandment of "And you shall love your friend as yourself." For if one truly loves his friend, he certainly will not speak lashon hara against him and he will seek merit for him with all of his strength. For if it happened to him that he did something unfitting and people were standing and talking about that thing and he knew of some aspect of merit in himself, whether it were his having acted unwittingly or the like, how much he would desire that there were someone who would speak up for him so that he not be unduly shamed. Exactly this should he do for his friend.

And this mitzvah of loving one's friend applies before the act, too, that his friend not come to shame. As when one marries off his son in a different city or when he himself comes to live in a different place, where he is a stranger to its customs. He taxes his brain to find some faithful friend there to apprise him of the customs of that place, and who, if he [the friend] finds him deviating from them in some way, will tell him so privately, so that he will take heed and not be embarrassed in the eyes of the men of the city. Thus, exactly, must he do for his friend. That is, if he sees him "stumbling" in some thing which is not to his credit, even if there is no issur in it, he must tell him so, that he not be shamed thereby. [And thus do we find in Horiyoth 13a with R. Yaakov b. Korshai and R. Shimon b. Gamliel. And more than this, we find in Sanhedrin 11a with several tannaim that they drew shame upon themselves to remove it from another. And how much more so if he sees him doing something against the din, he certainly must tell him so privately and reprove him for it so that he not fall into evil in the present and in the future; and he fulfils thereby the positive commandment of reproving and also the mitzvah of "And you shall love your friend as yourself."

And this will afford great pleasure to the Blessed Creator, his having perfected His son for His service. But if, G-d forbid, he does not do so, but, to the contrary, goes and shames him because of this before others, what does the Blessed Creator have from this?

To what may this be compared? To one who has a son who conducts himself improperly in some matter and his friend goes, in the eyes of his father, and publicizes this before others. Certainly, he [the father] will take offense and will say "You should have reproved him privately for this and I would have thanked you greatly for doing so, but you should not have gone and publicized him immediately for scorn and shame in the eyes of all. And I perceive that your intent was not concern for the truth. For if it were, it would have been better both for him and for you if you had reproved him for this. But your intent was only to blame and incriminate your friend and rejoice in his misfortune. This is nothing other than meanness of heart!"

So is it exactly with the Blessed Creator vis-à-vis the children of Israel, His holy people. For they are like sons to Him, as it is written (Devarim 14:1) "You are sons to the L-rd your G-d." And He rejoices in their good and grieves in their cries and in their shame (viz. Sanhedrin 46a). And this one [the friend] goes, in the eyes of the Father, whose glory fills all the world and shames His son before people to no purpose. What pleasure has He from this? And Rambam (Deoth 6:3) has adduced a principle in respect to the mitzvah of "And you should love your friend as yourself," that one should be just as solicitous of his friend's possessions and honor as he is of his own. And, as stated by Tanna d'bei Eliyahu 27: "Thus did the Holy One Blessed be He say to Israel: 'My beloved son, do I lack anything that I ask it from you? And what do I ask of you. Only that you love one another and honor one another and fear one another, and that there not be among you transgression and theft and unsightliness, so that you not be rejected forever, wherefore it is written (Michah 6:8): 'He has told you, O man, what is good. And what does the L-rd your G-d ask of you, but that you judge rightly, and love lovingkindness, and walk humbly with your G-d.'"