The occurrence of a miracle can be used as proof for the existence of the attribute of Speech. This is because the attribute of Speech is the one without which the expression of conventional signification (Dalala Wad’ia) would be impossible from God. And since the miracle is an expression of conventional signification from God, then the miracle is proof for the existence of the attribute of Speech for Him.
As for the attribute of Speech being necessary for the expression of conventional signification from God, we say: either God is attributed with Speech, or He is not. If He is not, then it makes no sense to claim that any expressions of conventional signification are affirmed from Him. After all, what is signified by this expression is all what is intended when we say “speech”. And the attribute by virtue of which God speaks is Speech. Thus, if an expression of conventional signification is affirmed from God, then the attribute of Speech must be affirmed for Him.
As for a miracle being an expression of conventional signification: then this is because God is the creator of normalcy. So His aiding a claimant to prophethood by negating normalcy for him, signals His support for this claimant. A miracle is as such, the equivalent of God’s saying: “this man tells the truth about Me.” More on this here.
Therefore, the occurrence of a miracle is proof that God is attributed with Speech.