This is the tribe which took its origin and had its name from Ishmael, the son of Abraham; and the ancients called them Ishmaelites after their progenitor. As their mother Hagar was a slave, they afterwards, to conceal the opprobrium of their origin, assumed the name of Saracens, as if they were descended from Sara, the wife of Abraham. Such being their origin, they practice circumcision like the Jews, refrain from the use of pork, and observe many other Jewish rites and customs. If, indeed, they deviate in any respect from the observances of that nation, it must be ascribed to the lapse of time, and to their intercourse with the neighbouring nations. Moses, who lived many centuries after Abraham, only legislated for those whom he led out of Egypt. The inhabitants of the neighboring countries, being strongly addicted to superstition, probably soon corrupted the laws imposed upon them by their forefather Ishmael. The ancient Hebrews had their community life under this law only, using therefore unwritten customs, before the Mosaic legislation. These people certainly served the same gods as the neighbouring nations, honouring and naming them similarly, so that by this likeness with their forefathers in religion, there is evidenced their departure from the laws of their forefathers. As is usual, in the lapse of time, their ancient customs fell into oblivion, and other practices gradually got the precedence among them.
Salminius Hermias Sozomenus, Ecclesiastical History, Book VI.
Tell me, you who are so eager to have the law for your master, have you never read the law? You will find it written there, that Abraham had two sons; one had a slave for his mother, and one a free woman. The child of the slave was born in the course of nature; the free woman’s, by the power of God’s promise. All that is an allegory; the two women stand for the two dispensations. Agar stands for the old dispensation, which brings up its children to bondage, the dispensation which comes to us from mount Sinai. Mount Sinai, in Arabia, has the same meaning in the allegory as Jerusalem, the Jerusalem which exists here and now; an enslaved city, whose children are slaves. Whereas our mother is the heavenly Jerusalem, a city of freedom. So it is that we read, Rejoice, thou barren woman that hast never borne child, break out into song and cry aloud, thou that hast never known travail; the deserted one has more children than she whose husband is with her. It is we, brethren, that are children of the promise, as Isaac was. Now, as then, the son who was born in the course of nature persecutes the son whose birth is a spiritual birth. But what does our passage in scripture say? Rid thyself of the slave and her son; it cannot be that the son of a slave should divide the inheritance with the son of a free woman.
— Epistle to the Galatians, iv. 21-30.