You have to remember that the son Abraham was to sacrifice wasn't the son that Abraham fathered from which Arabs are decended.
See #9. According to the Bible, the son in question was Isaac. But Muslimbs believe that Allah also ordered Abraham to sacrifice Ishmael.
It's one of a number of points on which the Qran and the basic teachings of Islam flatly contradict the Bible, which as far as I'm concerned makes Islam a heresy and a ripoff.