I've answered this question before here, which might be useful. I went through the earliest sources, but none of the ones I analysed viewed Islam as a heresy - instead the sources describe them as God's punishment, some sort of Judaising movement, or a broader Abrahamic group of some sort. The seventh century was an age of faith, so our sources, despite their limited number, are quite capable of noticing the difference between different beliefs. In the eighth century we do have a bold accusation by John of Damascus that Islam was a Christian heresy, but that was when Arab rule was much more entrenched and Islam itself more defined (there's a healthy academic debate right now about what exactly Islam was in the seventh century), but for the earlier period, I don't think Christians would have viewed it as a heresy. My answer on Christian reactions more generally to Islam should give you a a sense of the context, since it discussed the same sources further, which you might also find helpful :)