how did ishmael first learn about the incident on the town-ho?

This story is as told at the Golden Inn by Ishmael some time after the voyage on the Pequod. It is hence an interruption in the narrative . Three seamen from the Town-Ho know a secret and tell it to Tashtego, making him promise not to tell anyone. Tashtego talks in his sleep, and spills the whole story. The next morning he is forced to explain it to everybody.