Musaylima is the best attested of the several people who claimed prophetic revelation in Arabia in the years around 630, and he is also the one about whom almost nothing independent survives. His people, the Banu Hanifa, held the oases of al-Yamama in the centre of the peninsula, a productive agricultural district rather than a marginal one, and they were numerous enough to field an army that very nearly held.
The sources are the problem. Everything that reaches us about his teaching comes through Muslim historians and commentators writing from a century to three centuries later, in a tradition for which he is the type of the false prophet. His name is transmitted in a diminutive form, and Muslim scholars themselves recorded that his own people used the fuller name Maslama. Passages presented as his revelations are quoted in a manner designed to make them ridiculous beside the Quran, and no manuscript, inscription or non-Muslim notice offers a control.
This does not make him a fiction. The campaign against him is among the best documented events of the period, and the losses among Quran reciters at Aqraba are given in the tradition as the reason the collection of the Quran was undertaken at all. What cannot be recovered from the surviving record is the content of what he actually preached, and an honest account has to say so rather than repeat the parody.