Musar su Malachia 3:22

Also, from the Great Din, of which it is written (Malachi 3:19): "For, behold, the day is coming which will burn as an oven" — one who occupied himself with Torah is destined to be rescued, as we find in Midrash Tehillim: "In time to come, the Holy One Blessed be He will take the sun out of its case and judge [i.e., punish] with it the wicked, and heal with it the righteous, as it is written (Ibid. 20): 'And there shall shine for you, fearers of My name, the sun of charity with healing in its wings, etc.'" R. Yehoshua said: "The Holy One Blessed be He did charity with all who enter the world by not having placed it [the sun] in the first firmament. For if he had placed it there, there would be no shade for anyone under it, as it is written (Psalms 19:7): 'And there is no hiding from His sun.'" But in time to come, who is hidden from His sun? He who occupies himself with Torah, as it is written afterwards (Ibid. 8): 'The Torah of the L-rd is complete.'" And the verse (Malachi 3:19): "For, behold, the day is coming which will burn like an oven" is, likewise, followed by (Ibid. 22): "Remember the Torah of Moses, My servant."

The Ran proceeds to answer this question at length. What he says in essence is that Moses' speech defect was a necessary component of his very greatness. The Torah is called תורת משה; had Moses had a perfect command of speech, the greatness of Torah would not have become evident because people would have attributed the brilliance of the Torah's thoughts to Moses' eloquence. We all know that orators by their presentation can make mediocre content appear brilliant.

This paragraph of the Talmud is explained elsewhere as telling us that G–d has given us both a written and an oral Torah. The reason He did not write down the oral Torah was to provide the Jewish people with a סימן. Whereas the Gentiles copied the written Torah, G–d gave the Jewish people the oral Torah as a sign of distinction. This Torah is also called "Torat Moshe," since Moses handed it down to the Jewish people as הלכה למשה מסיני, as is, not subject to a variety of interpretations. When Maleachi, the last of the prophets, exhorts the Jewish people (Maleachi 3,22): "Remember the Torah of My servant Moses, which I have commanded to him at Chorev, statutes and social laws," he needed to exhort Israel to commit this to memory because it had not been recorded in writing.