Prophet Moses (peace be upon him) prayed to Allah for this because he was conscious that he was not a fluent speaker but was slow of speech. He also knew that as a Messenger he would have to be fluent in order to impress Pharaoh and his courtiers. This is confirmed by the Bible (Exodus 4: 10) but the Talmud gives a strange explanation for this impediment in his speech. According to it, when he was a child he burnt his tongue with a live coal to escape punishment from Pharaoh whose crown he had taken off from his head and put it on his own head. The same story has been adopted by many of our commentators though it is absurd on the face of it. Our interpretation is also confirmed by the Quran. The fact is, that at first he was not a fluent speaker and Pharaoh had also taunted him on this impediment. (Surah Az-Zukhruf, Ayat 52). Prophet Moses realized the same for he said: My brother Aaron is more fluent than me, so send him with me as a helper, so that he may support me (with his fluent speech). (Surah Al-Qasas, Ayat 34). Afterwards this impediment in his speech was removed and he was able to deliver very fluent speeches. This may be testified by his speeches which have been cited both in the Bible and in the Quran for they are masterpieces of eloquence and rhetoric.