What hint did Cedric give to Harry Potter in the Goblet of Fire during Triwizard Tournament about egg?

Cedric Diggory didn't give Harry any specific hint about the egg in the Goblet of Fire. He did, however, tell Harry to "listen carefully" after they both retrieved their eggs in the first task. This implied that the egg held a secret, and that Harry needed to pay close attention to it.

Cedric himself figured out that the egg held a clue to the second task, which was to retrieve something precious under water. He heard a mermaid singing inside the egg, which led him to the underwater solution.

Harry, on the other hand, missed this clue completely, and only solved the puzzle after a visit from Dobby, who revealed the secret to him.