1) He easily pronounces "Koran" in perfect, natural, native Arabic, while he can't even get "corps" right, as in Marine "corpse," and
2) He's referred to Islam as having been "revealed" to Mohamed. Nobody but a Muslim uses that term in referring to how Mohammed developed Islam, since use of that verb means the speaker believes Islam is the divine religion "revealed" to the world by Allah himself, a belief that necessarily makes the speaker a Muslim.
He also calls Jesus a “historical figure” (recognized by muslism, who deny He is the Son of God). He also says that Jesus is not the only (if any) way to salvation; Obama has said that he cannot picture God casting that many people out for following “false gods”.