While it's not a specific quote, the idea of guarding your mental space is a common theme in many books, including:
* Self-help and personal development books: These often emphasize the importance of controlling your thoughts and focusing on positive and constructive information.
* Spiritual and philosophical texts: These may explore the concept of mindfulness and the power of the mind to shape reality.
* Fiction: Authors may use metaphors like "guarding the portals of your mind" to illustrate the inner struggles of their characters.
Therefore, while the exact phrase "guard the portals of your mind" might not appear in a specific book, the underlying concept of mental control and mindfulness is a recurring theme in many different genres of literature.