Can anyone find or bring me a rayah from the gemara/rabbinical sources which implies/states explicitly that it Asur to tell Lashon harah to G-d?
כי מטא להדי פומא נח נפשיה משום דאמר ובתוך עם טמא שפתים אנכי יושב
Further, it seems likely that Y'sha'yahu was only talking to himself, not per se to HaShem (though it was in the presence of a vision of HaShem's presence).
Rashi on Exodus 4:3 quotes the Midrash as indicating that God was disapproving of Moshe's telling him lashon hara:
ויהי לנחש: רמז לו שסיפר לשון הרע על ישראל ותפש אומנותו של נחש:
and it became a serpent-: [This was how] He hinted to him [Moses] that he had spoken ill of Israel (by saying, “They will not believe me,” ) and he had adopted the art of the serpent. — [from Exod. Rabbah 3:12]
If someone can find this source. I saw it somewhere in the Sefer Shemiras HaLashon, brought down from an earlier source:
There is a pauk that states: אל תלשין עבד אל אדניו Do not slander a servant to his master. A generation that doesn't honor its mother and curses its father.
The commentary (Gemarah?)states: Even if a generation curses its father and does not bless its mother, don't slander them before its master, Hashem.
(May this answer also be l'ilui nishmas - גיל-עד מיכאל בן בת גלים, אייל בן איריס תשורה ויעקב נפתלי בן רחל דבורה)
The answer to your question, as noted by LN6595, is in the 7th chapter of "Sha'ar HaTevunah" in sefer "Shmiras HaLashon" (translation from http://shmirashalashon.blogspot.com/2006/12/shmiras-halashon-teves-6-ninety-sixth.html):
Wherein Shall be Explained the Greatness of being Careful to Avoid Arguing for the Guilt of a Jew, and the Reward for Arguing for the Innocence of a Fellow Jew
Behold, in the previous chapters, we explained regarding judging an individual person favorably. How much more so must a person be exceedingly careful to avoid arguing for the guilt of the Jewish People, for that sin is exceedingly great, as we find in Pesachim (87b): “What is the meaning of, 'A servant should not relate slander to his master… A generation that will curse its father and will not bless its mother' (Mishlei 30:10–11)?1 Even if the generation curses its father and does not bless its mother, one should not relate slander to his Master, The Holy One, Blessed is He.”
Please see regarding Yeshaya the Prophet: At the time that he beheld The Glory of HaShem, he said, “Woe to me, for I have been silenced, for I am a man of impure lips and in the midst of a nation of impure lips, do I dwell, for The King, HaShem Master of Legions, my eyes have seen” (Yeshayahu 6:5).
Because he said, “and in the midst of a nation of impure lips do I dwell”, even though he did not intend to scorn the Jewish People, for he also preceded by saying this in reference to himself. He only intended to say that he is not fitting to behold The Countenance of HaShem’s Presence, both due to his actions and due to the actions of the nation in whose midst he dwells. Nevertheless, see that which is written afterwards: “And one of the Serafim [angels] flew to me and in his hand was a cinder… 'notice that this has been touched upon your lips…'” (Yeshayahu 6:6–7)
Our Sages of Blessed Memory have said2 that the word “רצפה” – “ritzpah” [“cinder”] is an acronym for “רצץ פה” – “Ritzotz Peh” – “Crush the mouth that spoke slander against My children.” Eventually, Yeshayahu died as a result of this matter, as is written in Yevamos (49b)3: “…They brought the cedar tree and sawed it. Once they reached Yeshayahu’s mouth, he died”. See there.4
1 On this pasuk quoted from Mishlei, RaSh”I comments that one should not pass judgment against even a very sinful individual, the proof of this being the incident that the gemara records involving the Prophet Hoshea (Pesachim, 87a). Hoshea asked HaShem to exchange the Jewish People for another nation, as many of the Jewish People at that time were very sinful. In punishment for such a request, HaShem commands Hoshea to marry a woman who is promiscuous, who would bear him children of promiscuity.
3 The gemara (Yevamos, 49b) discusses the death of Yeshayahu the Prophet: King Menashe brought Yeshayahu before him with intent to kill him. In order to act as if his killing of Yeshayahu was justified, Menashe first queried Yeshayahu vis-à-vis some things which he had said, that, on the surface, may appear to contradict some things that Moshe Rabbeinu had said. Yeshayahu said that if he were to explain the verses that he was quoted as saying, as well as the ones that Moshe was quoted as saying, the apparent contradiction between Yeshayahu’s teachings and Moshe Rabbeinu’s teaching would dissipate. However, Yeshayahu said that if he were to indeed reconcile all of the verses quoting the two of them, Menashe would either have him killed for “intentionally” contradicting some of the teachings of Moshe Rabbeinu, or simply not accept what he were to say. Thereupon, Yeshayahu said one of HaShem’s Holy Names, and was swallowed into a cedar tree.
Menashe had the cedar brought to him, and had it axed open. Once they got to the point where they sawed Yeshayahu’s mouth, he died. The gemara explains that this was the punishment that befell Yeshayahu at the end of his life for saying (in reference to the Jewish People), “…and in the midst of a nation of impure lips do I dwell…” (Yeshayahu 6:5). RaSh”I (s.v. נסרוה) explains that Yeshayahu was punished for making the above statement in reference to the Jewish People, for HaShem did not command him to say it, nor did Yeshayahu make the statement for the purposes of rebuking the Jewish People.
For the rest of the chapter, which is relevant to this question: