How was Abraham able to keep the commandments, especially Rabbinical Laws, through the Torah wasn't given yet?
AskTheRabbi.org answered:
The Sages teach that Abraham was so finely in tune with God's Will that he was able to intuitively understand what was required of him even though the Commandments had not yet been given. So acute was Abraham's ability to seek out the correct actions that he was even able to understand what Rabbinic Laws were going to be added to the Laws that God gave and he lived his life according to both the Torah Laws and the Rabbinical Laws.