Was Islam originally considered to be a Christian sect? There is some speculation that Islam was originally considered to be a sect of Christianity by those who claimed to be Christians, rather than a separate religion (e.g., Hinduism, Jainism). Do any early sources during the formative years of Islam make such an assertion?

Muslims certainly don't believe Islam to be a Christian sect, but the Christians can easily fit it into the "Christian sect" fold as they have done it with the Mormons and Jehovah's Witnesses, since if Jehovah's Witnesses and Mormons can be classified as Christians then Islam also qualifies better for the Christians to qualify it as a Christian sect. So why keep Islam out?
Islam isn't, nor has it been considered a Christian sect, either by Muslims, or by Christians.

An article here argues that Islam is a Christian Heresy:


  Belloc states "It began as a heresy, not as a new religion....It was a
  perversion of the Christian religion...an adaptation and a misuse of
  the Christian thing."


But there is a distinction to be made. Further reading:


  Unlike all the other heresiarchs "[T]he chief heresiarch, Mohammed
  himself, was not, like most heresiarchs, a man of Catholic birth, and
  doctrine to begin with.  He sprang from pagans.  But that which he
  taught was in the main Catholic doctrine, oversimplified.  It was the
  great Catholic world - on the frontiers of which he lived, whose
  influence was all around him and whose territories he had known by
  travel-which


A Christian sect would be a sect that sprang out of Christianity at some point.  Islam does not meet this definition.  Instead, Mohammad borrowed some teachings from Christianity.

Or, if you want to phrase it differently, you could say that there is a commonality between Christianity and Islam, but that commonality is not based on Islam evolving from Christianity (as a sect would do) but rather that there is simply commonality due to similar beliefs.

As for the article saying that Islam is a Christian Heresy, it might be more accurate to state that Islam is partially based on Mohammad borrowing or agreeing with teachings that Christianity considers heretical.