"Abraham was ninety-nine years old when he circumcised the flesh of his foreskin, and his son Ishmael was thirteen years old when he was circumcised in the flesh of his foreskin. Thus Abraham and his son Ishmael were circumcised on that very day; and all his household, his homeborn slaves and those that had been bought from outsiders, were circumcised with him." Genesis.17.25
I've read some Kabbalistic explanations on why Abraham postponed his own circumcision till he was 100, but I didn't find why would he postpone circumcising all his male slaves on the day of purchase and Ishmael on the 8th day, given he kept all Biblical Laws before they were given.
So why did Abraham wait with Ishmael and the slaves?