Sforno on Exodus 6:12/ספורנו על שמות ו׳:י״ב

1הן בני ישראל לא שמעו אלי, he believed that this was due to the fact that from the moment he had started speaking to Pharaoh the latter had become worse in his attitude and the people, putting two and two together, had concluded that his mission had failed. If he felt that way, Pharaoh would certainly not pay any heed to him and continue doing what he liked.

2This was especially so, seeing that he was also an ערל שפתים, afflicted with a speech handicap. He mentioned this as he did not believe that Aaron would accompany him other than for the time he identified himself as G’d’s emissary when he spoke to the people.