Romans 3 clearly teaches he was justified when Genesis 15:6 says he was:

Romans 4:1-3 What shall we say then that Abraham our father, as pertaining to the flesh, hath found? For if Abraham were justified by works, he hath whereof to glory; but not before God. For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.

Abraham was justified before he was circumcised, before Isaac was born:

Romans 4:9-13 Cometh this blessedness then upon the circumcision only, or upon the uncircumcision also? for we say that faith was reckoned to Abraham for righteousness. How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision. And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also: And the father of circumcision to them who are not of the circumcision only, but who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised. For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith.