Biblically, circumcision is a sign of the Abrahamic Covenant between God and Abraham, and was incumbent on him and each of his (male) descendents lest they be "cut off from his people."
Insofar as they're descended from Ishmael, one could argue that Arabs (especially the prophet himself and his descendents) would be accounted as among Abraham's offspring and thus still affected by the Covenant. However, it would be harder to extend this same to non-Arab muslims who are not descended from the Abrahamic line (either by blood or by adoption).
To my understanding, circumcision is also considered mandatory in Shi'a Islam: Is there any evidence that this is directly due to the Covenant of Abraham, and if so, does it apply to even those muslims not of Abrahamic descent?
