EXOD75 Do not assume or exaggerate guilt. … Moshe's saying that the Jewish people would not believe him constituted loshon hara and for this Moshe was punished (See Rashi to verses 3 and 6). The Chofetz Chayim asked why Moshe's statement was considered loshon hora. It was necessary for Moshe to ask for a visible means of proving to the people that he was God's messenger. Why did God punish him for it? The Chofetz Chayim wrote in the name of Rabbi Eliyahu Shick that Moshe's error was in saying, "They will not believe me," which is an absolute announcement. Moshe should have said, "Perhaps they will not believe me," which would show that their skepticism was but a possibility. He had no right to assume that they positively would not believe him. Shmiras Haloshon 2:13. From here we learn that even if a person must say something against others for practical reasons, he must be careful not to exaggerate. (See Chofetz Chayim 10:2; see B'air Mayim Chayim 4:43 for a possible exception).