Actually, my theoretical quote is inaccurate. According to many commentators, Moshe probably said something like, "I'm n-n-n-n-n-not the guy. S-s-s-s-s-s-s-s-send s-s-s-s-s-s-s-omebody else." No, I'm not mocking Moshe. Chazal explain that Moshe was a stutterer.
When told to go lead the nation Moshe tells God, (Shemot 4:10)
What does this unusual phrase, כבד פה - "of heavy mouth" mean? Rashi writes,וַיֹּאמֶר מֹשֶׁה אֶל יְקֹוָק בִּי אֲדֹנָי לֹא אִישׁ דְּבָרִים אָנֹכִי גַּם מִתְּמוֹל גַּם מִשִּׁלְשֹׁם גַּם מֵאָז דַּבֶּרְךָ אֶל עַבְדֶּךָ כִּי כְבַד פֶּה וּכְבַד לָשׁוֹן אָנֹכִי:And Moses said unto the LORD: 'Oh Lord, I am not a man of words, neither heretofore, nor since Thou hast spoken unto Thy servant; for I am slow of speech, and of a slow tongue.'
(Just as an aside, Rashbam completely rejects this Midrashic interpretation, insisting that Moshe did not in fact suffer from a physical disability, but rather lacked the power of verbal eloquence, especially in the Egyptian dialect. Rashbam wonders: "Is it possible that a prophet who spoke to God face-to-face and received the Torah from [God's] hands stuttered in his speech?")בכבידות אני מדבר, ובלשון לעז בלב"א [גמגמן]"I speak with heaviness. In old French the word is "balba" - a stutterer
In fact, the issue of Moshe's speech limitations comes up again and again in Moshe's communications with God. In addition to his complaint here, he raises the issue of being an ערל שפתיים - "of uncircumcised lips" twice after he had already returned to Egypt! (see 6:12 and 6:30). It's clearly an issue that continued to concern him.
Yet, back at the bush, God rejects Moshe's concern, telling him:
How did God's answer address Moshe's issue? How would teaching Moshe what to say alleviate his concern over a physical speech impediment? Ibn Ezra explains:מִי שָׂם פֶּה לָאָדָם, אוֹ מִי-יָשׂוּם אִלֵּם, אוֹ חֵרֵשׁ אוֹ פִקֵּחַ אוֹ עִוֵּר--הֲלֹא אָנֹכִי, ה'. וְעַתָּה, לֵךְ; וְאָנֹכִי אֶהְיֶה עִם-פִּיךָ, וְהוֹרֵיתִיךָ אֲשֶׁר תְּדַבֵּר'Who hath made man's mouth? or who maketh a man dumb, or deaf, or seeing, or blind? is it not I the LORD? Now therefore go, and I will be with thy mouth, and teach thee what thou shalt speak.'
In other words, God promised to instruct him in the most effective way to speak in order to avoid the crippling effects of his speech impairment. While God might not have given Moshe therapy to improve his speech (and I hear it's doubtful whether there even is any effective therapy for stuttering), in giving Moshe strategies to improve his verbal communication, God was clearly acting as the world's first recorded speech therapist.וזהו טעם "אנכי אהיה עם פיך והוריתיך" - אמר שיורנו אשר ידבר מלות, שאין שם מאותיות הכבדות על פיוAnd this is the meaning of "And I will be with your mouth and I will instruct you" (verse 12) - that [God] would instruct him which words to speak, which did not have the letters that weighed upon his mouth