For the source text click/tap here: Taanit 27
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Reish Lakish said: They would not fast on Sunday due to the added soul, as Reish Lakish said: An added soul is given to man on Shabbat eve, and at the conclusion of Shabbat it is removed it from him, as it is stated:
“He ceased from work and rested [vayinafash]”
which he expounds as follows: Since one has rested and Shabbat has passed, woe for the soul [vai nefesh] that is lost, the added soul that each individual relinquishes. Consequently, one is still weak from this loss on Sunday.
We explore the mystical notion of the added soul of Shabbat.