I must confess that I copied this article on the Tzitzit from http://www.setapartpeople.com/tzit-tzit-a-reminder-throughout-our-generations . Its a good article that includes more details than I personally would bother with, but it does include the correct references to Abraham and his descendants.
Who are the descendants of Abraham?
Those who worship the same El as does Abraham. A key point is this: worship means we study His Word and apply His commands in our daily lives, and this includes all of the holy days established by Yahuah.