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Separating history from legend is often tricky. There is a legend stating that while Augustine of Hippo was writing his treatise De Trinitate (about the Trinity), he was walking along the beach and thinking about the Godhead. There he encountered a boy running back and forth between the ocean and a small hole in the sand, filling it with water. When Augustine asked the boy what he was doing, he replied, "I'm putting the sea into this hole." After Augustine commented on the absurdity of this notion, the boy retorted, "I will sooner empty the sea into this hole than you will manage to get the mystery of the Most Holy Trinity into your head!"
There is nothing in Augustine's writings containing this account, only legend. The story is told in the 15th century Legenda Aurea written by Jacobus de Voragine, and it is depicted in numerous works of art from that time until now.
It is possible that this legend influenced Tolstoy's telling of The Three Hermits, but that's somewhat of a stretch since these stories have different points they are trying to make. I am not aware of any other stories related to this associated with Augustine.