Here are couple of relevant quotes and VanDixhoorn's sources:
"In fact, Ali, Abdullah, Ubbay, and Hudaifa all held that parts of the Qur'an were missing, and the Mutazalite scholar, al Nazzam, accuses Abdullah of denying two Surahs that he held to be part of the Qur'an."
There's a footnote to John Burton's The Collection of the Qur'an, 124, 130-131, 117 and 200.
"Jeffrey's work is basically a catalogue of textual variations (comparing today's text with the oldest sources) from 15 primary codices, 13 secondary codices, and some unnamed codices. He sees this as the first step towards a critical text of the Qur'an (similar to the critical texts of the Bible). His work has not made Muslim apologists particularly happy, though Von Denffer acknowledges the usefulness for his work."
The reference there is to Arthur Jeffrey's The Qur'an as Scripture.