It is repeadly told you that it is allah. Gabriel is the messenger off god. Gabriel said the words which god spoke to him and spoke those to muhammad. It was imprinted upon muhammad's mind so he could tell his firends who could read and write. To have gabriel do it as well, and still have muhammad believe in them being the words of allah is proff of faith, adn a test of faith. Muhammad did at first think "is this readl or am i crazy" but he realised he wasn't. The Qur'an is perfect, no one else can replicate it, therefore that alone should be evidence enough of it being the word of god, it was for me.
People don't say muhammad said "....blah blah, qur'anic vrse this, line that...", they say allah said in qur'anic verse this and line that, or in the qur'an it says this and that. For hadith and sunnah it says, related by soand so, the messenger of allah said this and that.