"Nothing prevents God from sending into His world whomsoever He wishes, since the world of holiness sends forth emanations unceasingly... Even before the revelation of the Law he sent prophets to the nations... and again after its revelation nothing prevented Him from sending to them whom He wishes so that the world might not remain without religion... Muhammad was a prophet to them but not to those who preceded them in the knowledge of God. He permitted to every people something He forbade to others. He sends a prophet to every people (Qur'aan 14.4) according to their language."
Ibn al-Fayyumi, the Bustan al-Ukul (ed. and trans. D. Levine; New York: Columbia University Press, 1908; repr. 1966), pp. 2 and el-Uqul: Gan ha'Sekhalim (Jerusalem: Halikhot Am Israel, AM 5744/1984)