Islam includes the sunnah of Muhammad
Quran only Muslims would disagree. Quranism - Wikipedia
How would that be 'the same' religion though? If you took Christianity and eliminated half the practices and beliefs, it would no longer be Christianity.
What practices and beliefs? Shi'a, Sunni and other groups have different beliefs and practices to a greater or lesser degree. Sufis who are aligned with Islam often note that the Quran has an inner meaning distinct from the outer meaning.
Early Christians worshiped in houses not churches. Eastern Orthodox, Catholic and Protestant Christians have very different practices.
And it was not until the Council of Nicea and the Nicene Creed that beliefs were codified.
To use Judaism as another example, what are the "practices and beliefs" of practitioners of Judaism from ultra Orthodox to Reform. And where does the belief in reincarnation from Kabbalah fit in with Jewish belief?