So Sarah dies, Abraham needs to buy a burial plot for her, and he asks for a cave in a field belonging to Ephron the Hittite. What did Abraham know about that cave that he was willing to pay 400 shekels for it?
Allow me to share some history as it is recorded in the mystical texts.
Rabbi Rechumai taught that after Adam was banished from the Garden of Eden, G‑d Himself hewed out of rock a place near the entrance to the Garden where he would bury Adam, the patriarchs and the matriarchs.
(Rabbi Kisma added that when Eve died and Adam buried her there, he smelled the tantalizingly familiar aroma of the Garden and wanted to burrow further, but a voice rang out telling him to stop. When Adam died, he was buried there as well.)
Eventually the location was forgotten, and not a soul knew about it. Until Abraham, who entered the cave, saw it, sniffed the scent of Eden, and heard a voice saying, “Adam is buried here, and Abraham, Isaac and Jacob should be prepared for this place as well.” He saw a candle burning there, and left.
From then on, he had his heart set on that special burial plot. Many others attempted to be buried there, but the ministering angels guarded the place faithfully. The would-be buriers would see the fire burning and were unable to enter—until Abraham entered and purchased it.
How did Abraham come across the cave in the first place?
Rabbi Elazar revealed that Abraham first found this special place on the day that the angels came to tell him that Sarah would give birth to Isaac. When the three men (angels) visited Abraham, he chased after a calf to slaughter and feed his guests (see Genesis 18:7). The calf ran into the cave, and Abraham followed in pursuit. After that, he would go every day to that field from whence the heavenly aromas came. There he would pray to G‑d, and there G‑d would speak to him.
Now, the nature of the cave was such that only its true owner was able to see its special qualities. Where Abraham saw light, Ephron saw darkness.
Nevertheless, Abraham did not purchase the place until Sarah passed away, so as not to arouse people’s interest in the place. Only when he needed to bury his beloved wife did he buy Ephron’s field and its wondrous cave.
Zohar Chadash, Ruth; Zohar 1:127–128.