The Burial:

Soul does not move on until burial:^[1] The Zohar[2] states as follows: The soul cannot continue with its journey, either in Gan Eden, or as a Gilgul back in this world, until the body is buried. This is because a new body cannot be given to the soul until his old body is buried. It is possible that G-d predestined this soul to be reincarnated in a body that very day of his death, and the delay of the burial can cause a delay in this reincarnation. It is similar to a man who lost his wife and cannot marry a new wife until his late wife is buried. For this reason, the soul also cannot be elevated to Gan Eden until the body is buried, as to enter Gan Eden one needs to receive a spiritual body, and this body cannot be given until the original body is buried. It is therefore a Mitzvah to bury the deceased right away.

Effect on soul if buried in Eretz Yisrael:[3] One who is buried in Eretz Yisrael is saved from Chibut Hakever, and from infestation. The soul goes straight up to heaven, as Chazal state[4] that all the souls are elevated through Yerushalayim.[5] See Chapter 4 Halacha 5C!

Effect on soul if buried near a Rasha: One may not bury a Rasha near a Tzadik[6], as the soul of Tzadik is pained if it is buried near a Rasha.[7] See Chapter 4 Halacha 5D!

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[1] Mavor Yabok Mamar Sefas Emes 27; Zohar Parshas Emor p. 88b

[2] Parshas Emor p. 88b

[3] See Gesher Hachaim 27:8

[4] Midrash Raba Vayikra

[5] Gesher Hachaim ibid

[6] Michaber 362:5; Mishneh Sanhedrin 46a and Gemara 47a

Is this Biblical, Rabbinical, or a custom? Some Poskim rule that this law is a Biblical obligation, and a tradition of Moshe from Sinai [Chasam Sofer 341; Minchas Elazar 2:41; Igros Moshe 2:152; Teshuvos Vehanhagos as so is learned from the above Mishneh regarding Harugei Beis Din] Others rule that this law is a Rabbinical obligation. [Dvar Mishpat] Others rule that this law is a mere custom. [Maharil Semachos 10]

Other opinions: Some Poskim rule that it is permitted to bury a Tzadik near a Rasha and the ruling in the above Mishneh is limited to only Harugei Beis Din. [Chasam Sofer ibid in opinion of Rambam who omitted this Halacha]

[7] Story brought in Sefer Chassidim 705 that it is like being buried near a bathroom that has a foul smell; Maharil ibid that the soul of the Tzadik is not revealed the secrets of heaven if he has a Rasha neighbor.