Tamid 28a: One who rebukes someone l'Shem Shamayim merits Hashem's portion -- "Mochi'ach Adam Acharai." Furthermore, a string of Chesed is drawn over him -- "Chen Yimtza mi'Machalik Lashon."

Rashi: He rebukes someone for his Aveiros, and separates him from them. Afterwards, he will find grace in his eyes more than a flatterer. Wherever it says Acharei, this is after a long time. Wherever it says Achar, this is after a short time (Bereishis Rabah 44:5).

Rashi (from Tanchuma ¹ Devarim Hosafah 2): Mochi'ach Adam is Moshe, who rebuked Yisrael, who are called Adam - "Adam Atem" (Yechezkel 34:31). It says about Moshe "Matzasa Chen b'Einai" (Shemos 33:12). Machalik Lashon is Bil'am, who said proper words to Yisrael - "Mah Tovu Ohalecha Yakov" (Bamidbar 24:5). Moshe announced 'beware, lest you be enticed!' Bil'am announced and flattered 'do your desires, and He will not be adamant.' "Lo Ish Kel va'Ychazev" (Bamidbar 23:19), for the nations (He will punish like He said), but for you (Yisrael) "Amar v'Lo Ya'aseh" (ibid). Shlomo commanded about both of them - "Ne'emanim Pitz'ei Ohev v'Ne'etaros Neshikos Sonei" (27:6).

Malbim: Even though the way of the world is that one who does not rebuke people for their sins finds grace in their eyes, and all the more so if he flatters and praises their deeds. If a Chacham sees that people of his city love him