Talmud Bavli Bava Batra on the 'Double' Cave of Mechpelah

"Rabbi Bana'ah would mark the boundaries of burial crypts, so that people would not inadvertently walk over them and contract tumah (ritual impurity). When he reached the crypt of Abraham and sought to enter in order to measure its dimensions, he found Eliezer, the servant of Abraham, standing at the front of the entrance. Rabbi Bana'ah said to him, 'What is Abraham doing?' He replied, 'He is lying in the arms of Sarah, and she is peering at his head.' Rabbi Bana'ah said, 'Go and tell Abraham that Bana'ah is standing at the entrance.' Abraham said to him, 'Let him enter. It is well known that there is no physical desire in this world.' Rabbi Bana'ah entered the crypt, surveyed its dimensions and departed.

"When he reached the nearby crypt of Adam, and attempted to enter to measure its dimensions, a Heavenly voice came forth and proclaimed, 'You have gazed at the likeness of My image [i.e. Jacob]. Do not gaze at My image itself [i.e. Adam]. Rabbi Bana'ah replied: 'But I want to mark the extent of the crypt in order to ascertain the precise dimensions.'

"The heavenly voice responded, 'As the dimensions of the outer crypt (where the Patriarchs and Matriarchs were buried), whose size you have already determined, so are the dimensions of the inner crypt, in which Adam is buried. There is no need for you to enter and measure. And according to the one who says that the 'double' cave of Machpelah was configured as two rooms, one atop the other: As the dimensions of the upper room, in which the Patriarchs and Matriarchs are buried, so are the dimensions of the lower room, in which Adam is buried. Either way, no additional measurements need be taken.'

"Rabbi Bana'ah said, 'I glimpsed Adam's two heels from outside the crypt, and they were like the two orbs of the sun."

-- Talmud Bavli, Bava Batra 58a