In Parshat VaEira (9.35) it says "b'Yad Moshe" כַּֽאֲשֶׁ֛ר דִּבֶּ֥ר יְהֹוָ֖ה בְּיַד ־משֶֽׁה ; so what was the *hand* of Moshe that the Torah tells us here about? 

(And Pharaoh's heart was hardened, and he did not let the children of Israel go out, as the Lord had spoken (debeer) through the hand (b'Yad Moshe) of Moses.)

Did Moshe's speech impediment force him to speak *sign language* so that Ahron would understand him?