Here's how it happens:
* Utterson receives a letter from Hyde, who is fleeing after the murder of Sir Danvers Carew.
* Utterson is disturbed by the letter's content and compares it to a note Dr. Jekyll had previously given him.
* He notices that the handwriting is identical.
* This discovery deepens his suspicion that Dr. Jekyll is somehow connected to Hyde, setting him on a path to uncover the truth.
This moment is crucial in the plot, as it reinforces the reader's suspicions and sets the stage for Utterson's further investigation.