The page number of the quote will vary depending on the edition of the novel. This quote takes place towards the end of part one, when Montag entertains the idea that there may be something important and influential inside books. Montag has recently witnessed a woman commit suicide with her library, and the entire ordeal makes him seriously question his unfulfilling occupation. Montag then argues with Mildred about going to work and tries to explain his complex emotions by saying,

Last night I thought about all the kerosene I've used in the past ten years. And I thought about books. And for the first time I realized that a man was behind each one of the books. A man had to think them up. A man had to take a long time to put them down on paper. And I'd never even thought that thought before.

This quote can be found on page 25 of the online pdf. edition of Fahrenheit 451, which is included in the reference link at the bottom of the question. Montag's comments illustrate his remorse for destroying works of literature that have taken some men their entire lives to complete. He is finally grasping the enormity of his destructive occupation and desires to quit immediately. He is hopelessly attempting to make Mildred understand his decision to begin reading books.

Living in a dystopia, where literature and knowledge are censored, is negatively affecting Montag's emotional and intellectual development. He is beginning to question the authoritarian government's stance on literature and having second thoughts about his occupation. Shortly after Montag expresses his remorse for destroying books his entire career, Captain Beatty arrives and proceeds to lecture him about the importance of the fireman institution.

On page 49 of the 60th Anniversary Edition of Fahrenheit 451 published by Simon & Schuster, Montag refuses to go into work and tells his wife,

"Last night I thought about all the kerosene I've used in the past ten years. And I thought about books. And for the first time I realized that a man was behind each one of the books" (Bradbury, 49).

In Part One of the novel, Montag meets his extrovert neighbor Clarisse, who asks him if he is happy with his life. Clarisse then tells Montag that he is not in love, and Montag begins to contemplate his meaningless life. Montag realizes for the first time that he is living an unfulfilling, unsatisfying life and wishes to make a change. As he lays in bed, he thinks about his occupation, stagnant relationship, and meaningless life. Montag then sits up in bed and begins explaining his feelings to his superficial wife. Before Captain Beatty arrives at his home, Montag tells his wife that he refuses to go into work and is fascinated by the possibility that the answers he is seeking may be inside of the same books he has been burning over the past ten years.

The quote that you are referring to starts on page 51 and continues on the top of 52.  I have the 50th anniversary edition of the book.

The quote comes from a conversation that Montag is having with his wife Mildred, when he has brought books into their home and he is trying to convince her of their importance.

"And for the first time I realized that a man was behind each one of the books.  A man had to think them up.  A man had to take a long time to put them down on paper." (Bradbury)