Adam Clarke on Genesis 15:15

Thou shalt go to thy fathers in peace - This verse strongly implies the immortality of the soul, and a state of separate existence. He was gathered to his fathers - introduced into the place where separate spirits are kept, waiting for the general resurrection. Two things seem to be distinctly marked here:

1. The soul of Abram should be introduced among the assembly of the first-born; Thou shalt go to thy fathers in peace.

2. His body should be buried after a long life, one hundred and seventy-five years, Gen 25:7. The body was buried; the soul went to the spiritual world, to dwell among the fathers - the patriarchs, who had lived and died in the Lord. See note on Gen 25:8.