From his earliest youth Mohammed was addicted to religious contemplation; each year during the month of Ramadan, he withdrew from the world and from the arms of Cadijah; in the cave of Hera, three miles from Mecca, he consulted the spirit of fraud or enthusiasm, whose abode is not in the heavens, but in the mind of the prophet. The faith which, under the name of Islam, he preached to his family and nation, is compounded of an eternal truth and a necessary fiction, THAT THERE IS ONLY ONE GOD AND THAT MOHAMMED IS THE APOSTLE OF GOD.[ emphasis mine]
The decline and fall pg 458
It is expected that he [Gibbon] being religious would say it is an eternal truth that there is only one god, but leaving that out, the rest of the passage is spot on.