
The Citadel stripped me of it.
Game of Thrones Season 3, Episode 5

Brought shame on the Citadel
with his repugnant experiments.
Game of Thrones Season 4, Episode 2

You don't get thrown out of the Citadel
without good reason.
Game of Thrones Season 6, Episode 3

A maester of the Citadel, chained and sworn,
Game of Thrones Season 5, Episode 7

So, why did the Citadel take your chain?
Game of Thrones Season 3, Episode 7