He had a stutter. That is correct. A du’a that prophet Musa pbuh made when proceeding to the court of Fir’awn (Pharoah) for the arguments (inviting him to Islam). Rabb-ishrah li sadri, wa yassir li ‘amri, Wah-lul ‘uqdatam-min-li-saani, Yaf-qahuu qawlii O my Lord! Open for me my chest (grant me self-confidence, contentment, and boldness); Ease my task for me; And remove the impediment from my speech, so they may understand what I say [Surah Ta-Ha; 20:25-28]