What is the first mention of Harry's magic - Answer

His hair growing back after being cut.

Once, Aunt Petunia, tired of Harry coming back from the barbers looking as though he hadn't been at all,had taken a pair of kitchen scissors and cut his hair so short he was almost bald except for his bangs, which she left "to hide that horrible scar." ... Next morning, however, he had gotten up to find his hair exactly as it had been before Aunt Petunia had sheared it off. He had been given a week in his cupboard for this, even though he tried to explain that he couldn't explain how it had grown back so quickly.