והנראה אלי מדעת רבותינו שלמד אברהם אבינו התורה כלה ברוח הקדש ועסק בה ובטעמי מצותיה וסודותיה ושמר אותה כולה כמי שאינו מצווה ועושה ושמירתו אותה היה בארץ בלבד ויעקב בחוצה לארץ נשא האחיות וכן עמרם כי המצות משפט אלהי הארץ הן אף על פי שהוזהרנו בחובת הגוף בכל מקום (קדושין לו) וכבר רמזו רבותינו הסוד הזה (בספרי דברים פסקא מג) ואני אעירך בו בעזרת השם (ויקרא יח כה דברים יא יח)
And what appears to me from the opinion of our rabbis is that Avraham learned the entire Torah from holy inspiration, and was occupied it its [study] and in the reasons for its commandments and its secrets, and he kept all of it as one who is not commanded and does. And his observance of it was only in the Land [of Israel]. And Yaakov married two sisters outside of the Land, and also Amram [was outside the land when he married his aunt], since the commandments are 'the law of the God of the land,' even though we are commanded [them] as an obligation on ourselves in every place (Kiddushin 36). And our rabbis already hinted to this secret (Sifrei, Devarim 43) and I will write about this at length (Ramban on Leviticus 18:25 and Ramban on Deuteronomy 11:18) with God's help.
...וְלָמָּה הִקְדִּים הַכָּתוּב מִיתָתוֹ שֶׁל תֶּרַח לִיצִיאָתוֹ שֶׁל אַבְרָם? שֶׁלֹּא יְהֵא הַדָּבָר מְפֻרְסָם לַכֹּל וְיֹאמְרוּ לֹא קִיֵּם אַבְרָם אֶת כְּבוֹד אָבִיו, שֶׁהִנִּיחוֹ זָקֵן וְהָלַךְ לוֹ, לְפִיכָךְ קְרָאוֹ הַכָּתוּב מֵת
...Why, then, does Scripture mention the death of Terah before the departure of Abram? In order that this matter (his leaving home during his father’s lifetime) might not become known to all, lest people should say that Abram did not show a son’s respect to his father, for he left him in his old age and went his way. That is why Scripture speaks of him as dead (Genesis Rabbah 39:7).