והשענו תחת העץ. This teaches that there was only a single tree at that location. Why did G’d reveal Himself in the shape of a tree? This was because He told him that he would sire a son after having become old, similar to a tree of which it is written that even “an old tree still has hope of producing new branches even if it has been cut down, it will renew itself, its shoots will not cease, if its roots are old in the earth and its stump dies in the ground. At the scent of water it will bud and produce branches like a sapling.”(Job 14,7-9) Furthermore, the righteous have been compared to trees, as we know from Psalms 1,3 והיה כעץ שתול על פלגי מים, “he will be as a tree planted besides streams of water,” (quoted by Rabbeinu Bachya in the name of Rabbeinu Chananel in his commentary on Genesis 18,1).

למען אשר יצוה את בניו ואת ביתו אחריו, the dictates by the patriarchs to their respective children, all only by word of mouth, without there being a written document claiming to be G’d’s instructions such as the Torah nowadays, were observed by these children of the patriarchs as if the Torah had already been given, and as if they each had said: “I will do and I will hear.” This is further proof that the oral Torah is not subordinate to the written Torah. All of what was written in the written Torah was included in what the people heard at Mount Sinai, as well as all that G’d taught Moses while he was on that mountain. Just as there is no difference between the specific commandment the people had heard, i.e. the Ten Commandments, and the remainder of the written Torah, so there is no difference [in the authenticity and binding nature Ed.] of the commandments not spelled out in the written Torah, but handed down by the sages orally from generation to generation. (Rabbeinu Bachya)