Surah 20. Ta-Ha, Ayah 28

I.e., "remove all impediment from my speech" (cf. Exodus iv, 10, "I am slow of speech, and of a slow tongue"), which would imply that he was not gifted with natural eloquence.

 Moses (ﷺ) put a brand of fire in his mouth, which hindered his speech as he grew up. In this verse, he prays to Allah to help him speak clearly, and his prayer is answered.