Principle 7 8from Chafetz Chaim

וַאֲפִלּוּ בִּכְגוֹן מַעֲשֵׂה דְּטוּבִיָּה הַנַּ"ל, אֵינוֹ מֻתָּר לְקַּבֵּל סְתָם, רַקּ בְּהִצְטָרֵף אֶל זֶה שְׁנִי הַפְּרָטִים, שֶׁצָּרִיךְ לִזָּהֵר בָּהֶם בְּעִנְיַן הַקַּבָּלָה.

And even [if what is spoken of is like the act of Toviyah (viz. Principle VII)], it is not permitted to be accepted categorically, but only with the two following qualifications:

א דַּוְקָא אִם סִפֵּר לוֹ, שֶׁרָאָה דָּבָר זֶה בְּעַצְמוֹ, אֲבָל אִם (יד) שָׁמַע מִפִּי אֲחֵרִים, אֵין לְהַמְסַפֵּר הַזֶּה שׁוּם יִתְרוֹן כְּלָל.

a) only if he [the speaker] tells him that he himself saw the thing; but if he heard it from others, this speaker has no advantage at all.

ב אֲפִלּוּ אִם סִפֵּר לוֹ, שֶׁרָאָה בְּעַצְּמוֹ, אֵין מֻתָּר, רַק לְהַאֲמִינוֹ וּלְהַרְחִיק אֶת עַצְמוֹ מֵחֶבְרָתוֹ, עַד אֲשֶׁר יִוָּדַע, שֶׁשָּׁב מִדַּרְכּוֹ הָרָעָה, אֲבָל לֹא לֵילֵךְ וּלְסַפֵּר דָּבָר זֶה לַאֲחֵרִים, וּכְמוֹ שֶׁכָּתַבְנוּ לְעֵיל בִּכְלָל ד' סוֹף סָעִיף ה', וּמִכָּל שֶׁכֵּן שֶׁאֵין (טו) לְהַפְסִידוֹ בְּמָמוֹן אוֹ לְהַכּוֹתוֹ, חַס וְשָׁלוֹם, עַל יְדֵי זֶה.

b) Even if he told him that he saw it himself, it is permitted only to believe him and to distance himself from his [the sinner's] company until it is known that he repented of his evil way, but not to go and reveal it to others (viz. PrincipleIV, end of section 5). How much more so is it not permitted to cause him monetary loss or to strike him, G–d forbid, because of this.