Summary: One who drinks water on the eve of Shabbat or, according to other opinions, following mincha on Shabbat, steals water from the souls in Gehenom and thus harms them.
There is little, if any, Scriptural discussion regarding the continued life of a soul following death, therefore we are left exploring the murky depths of Midrash. I refer you to the (odd?) halachic dispute regarding drinking water on the eve of Shabbat and following mincha on Shabbat. A good discussion of the argument can be found in the book 'Kaddish' by Leon Wieseltier. Here are some excerpts [page 100-105 of first Vintage Books edition, 2000]:
The ערוגת הבושם:
.... on the Sabbath the dead are relieved of the judgment of hell. On
that afternoon the souls are made to stand by a gleaming fountain of
water that flows at the entrance to the garden, and then rinse
themselves in the water to cool their bodies from the fire... Since
the souls at that hour are standing by that fountain, the geonim and
the [post-Talmudic] rabbis established the custom that we do not drink
water between the afternoon service and the evening service on the
Sabbath, because we would be stealing it from the dead."
The שיבולי הלקט [about]:
According to a rabbinical legend, when an individual drinks water at
twilight [on the afternoon of the Sabbath], it is as if he were
stealing the water from his dead. And I have found this in the
responsa of the geonim: 'We have heard it said in the name of the
early sages that for the duration of twilight, permission is granted
to the souls of the dead to drink water. And when one drinks water at
the hour when the souls of the dead are drinking water, the souls of
the dead who are one's kin are not permitted to drink. For this
reason, the sages said that it is as if one were stealing from one's
own relatives."
The אור זרוע from Isaac of Vienna:
When the afternoon [of the Sabbath] comes... it is the practice of the
righteous.... and then to eat a meal. Our Master Tam [רבנו תם] scolded
them and said that it was forbidden, citing the tale in the Jerusalem
Talmud about a man who drank water between the afternoon prayer and
the evening prayer, and the angel of death came and slew him, because
he drank when the dead were drinking and so was a robber of the dead."
The מרדכי from Mordecai ben Hillel:
... Rabbi Meshullam responded that in his text of the ancient
rabbinical legend he had a different formulation. His text said that
'those who eat and drink on the afternoon of the eve of the Sabbath
[steal from the dead] and the reason is that [by Friday afternoon] the
dead are exhausted from the judgment they have endured all week. And
for this reason he was strict about not drinking on the afternoon of
the eve of the Sabbath.... And it is the custom of the whole world to
be careful [about drinking the water of the dead]..."
The Bach concludes that one should not drink either Friday afternoon or Saturday afternoon.
Needless to say, the practice of refraining from drinking during these times has all but disappeared, though as can be seen above, was widely practiced in Ashkenazic lands in the Middle Ages.
