Maḥzor Vitry: On the Conclusion of the Sabbath

Thus we lengthen the prayers at the conclusion of the Sabbath, saying Psalms 90:17–91:16, Seder Kedushah [a version of the prayer known as the sanctification of the name], and verses of comfort, for on the Sabbath day the sinners among Israel leave Gehenna and rest, as it is said in the chapter of Rabbi Akiva [see, instead, b. Sanhedrin 65b]: [They do not return] until Israel completes the evening prayers, so we lengthen the prayers so that they do not return too quickly to Gehenna.

I found in the responsa of the geonim, concerning the rabbinic statement that whoever drinks water at twilight is like one who steals from his deceased relatives [see the Midrash on Psalms 11:6], that at twilight they all drink. Within twelve months of their passing, the souls are given permission to drink water, and when the living relative drinks water at twilight, the other souls do not allow the soul of his relative to drink. They said this only with respect to the seven relatives for whom a priest makes himself impure [in order to bury them]: his father, his mother, his brother, his sister, his son, his daughter, and his wife, and only within the first twelve months when the body endures, and the soul ascends and descends into the body. But from twelve months on, there is no concern.