TT #4 · Parashas Chayei Sarah · Passuk 23:4
“I am a resident alien among you; sell me a burial site among you, that I may remove my dead for burial.”
Torah Temimah points to Berachos 18a (and related). E-Daf · Sefaria — Berachos 18a
TT lemma: ואקברה מתי מלפני.
Rav Ashi said: Once it is incumbent upon a person to bury the deceased, it is considered as though the deceased is lying before him. This is derived from “Abraham rose from before his dead” and “that I may bury my dead from before me”; as long as he is responsible for the burial, it is considered as though the deceased is before him.
Custom (Bavli — onen exemption and equivalence)
An onen whose deceased is his responsibility is exempt from positive mitzvos even when the body is not physically present, because the burial obligation itself renders the case “before him.”