Salaam!
The Quran states clearly that what was being sent down to the Messenger was nothing else but the Quran;
Ha Mim: A REVELATION from the Beneficent, the Merciful Allah: A BOOK, whereof the Ayat are explained in detail; - a QURAN in Arabic, for people who understand - (41:1-3).
The Messenger himself is made to confirm that the Wahi he is receiving is the Quran:
‘And this Quran has been revealed to me [uhiya ilayya ha_zal Qur'an] that with it I may warn you and whomsoever it reaches?’ (6:19)
Even hypocrites were afraid that a Surah might be revealed to show their hypocrisy. The Prophet’s ordinary speech was always present, but the Quran tells us that it was a Surah that they were afraid would reveal their hypocrisy:
‘The hypocrites fear lest a Surah (chapter of the Qur'an) should be sent down to them telling them plainly of what is in their hearts. Say: Go on mocking, surely Allah will bring forth what you fear?’ - (9:64)
Moreover;
‘O you who believe! Do not put questions about things which if declared to you may trouble you, and if you question about them when the Quran is being revealed, they shall be declared to you; Allah pardons this, and Allah is Forgiving, Forbearing.’ -(Ch.5: Ver.101)
The above restriction would have been meaningless if Prophet had received other revelations besides the Quran. This proves that there were no other revelations besides the Quran.
In addition to the Qur'anic evidence, even the books of Hadith also refute this baseless claim that the Messenger was receiving other revelations besides the Qur'an:
Bukhari: Volume 4, Book 52, Number 283: Narrated Abu Juhaifa: I asked Ali, Do you have the knowledge of any Divine Inspiration besides what is in Allah's Book?" 'Ali replied, "No, by Him who splits the grain of corn and creates the soul. I don't think we have such knowledge, but we have the ability of understanding which Allah may endow a person with, so that he may understand the Qur'an, and we have what is written in this paper as well. I asked, what is written in this paper? He replied, (The regulations of) blood-money, the freeing of captives, and the judgment that no Muslim should be killed for killing an infidel.
The claim of un-recited revelation to prophet is a fabrication. Such a view is deliberately promoted to justify collection of hadiths and its authenticity. The Quran does not support such fabrications.
Regards,
Optimist