Who sounded the alarm in Fahrenheit 451 by Ray Bradbury?

Montag’s wife Mildred sounded the alarm on him.

In Montag’s community, it is illegal to have books in your possession.  They take this law so seriously that if you are caught with a book, your house will be burned.  Houses are fireproof, and the books and everything else burn but the house does not.

Montag is a fireman, and he is curious about books.  During one raid, he steals a book and shows it to his wife.  She doesn’t understand the appeal.  Mildred is like most people in Montag’s community.  She is more interested in the “relatives” on television.  She wants a huge four walled TV set and cares about little else.  She and Montag lack an emotional connection.

Montag’s boss Beatty is suspicious.  He warns Montag that books are not all they are cracked up to be.  Not believing him, Montag seeks out a retired professor to ask about the world of books.  Faber seeks to help him navigate avoiding suspicion.  Unfortunately, one day an alarm is sounded and it turns out to be Montag’s house.

Beatty, smelling of the wind through which he had rushed, was at Montag's elbow. "All right, Montag?"

… At last Montag raised his eyes and turned. Beatty was watching his face.

"Something the matter, Montag?"

"Why," said Montag slowly, "we've stopped in front of my house." (Part 2)

Montag is horrified and frightened, but Beatty is enjoying it.  His boss is dragging it out, having Montag like a fish on a hook.  Mildred comes running out of the house, and shockingly is more concerned about the house and the fake people on TV than her husband.

"Mildred! "

She ran past with her body stiff, her face floured with powder, her mouth gone, without lipstick.

"Mildred, you didn't put in the alarm!"

She shoved the valise in the waiting beetle, climbed in, and sat mumbling, "Poor family, poor family, oh everything gone, everything, everything gone now ...." (Part 3)

Montag is trapped.  Like a caged animal, he can only react.  He attacks Beatty, killing him with a flamethrower.  He is worried about the Mechanical Hound, which will now hunt him down.  Betrayed by his wife, Montag goes on the run.