This question already has an answer here:
Why doesn't Harry get in trouble for casting Lumos (and thereby breaking The Decree for the Restriction of Underage Wizardry) AFTER he flees the Dursley's house when he first sees Sirius in his animagi form right before The Knight Bus arrives?
(It happens on page 33, in Chapter 3 of my copy of the Prisoner of Azkaban)
Is this maybe pardoned by Fudge due to the possibility that Voldemort or Sirius might have attacked Harry? At the bottom of page 43, Fudge implies that he had thought something bad had happened to Harry.