Прогнал поиск по всем семи книгам — похоже, первое упоминание как раз в 6-й книге и есть. По некоей иронии — тоже в 11-й главе :)

"We've got to go and explain," said Hermione, looking up at Hagrid's huge empty chair at the staff table the following Saturday at breakfast.
"We've got Quidditch tryouts this morning!" said Ron. "And we're supposed to be practicing that Aguamenti Charm from Flitwick! Anyway, explain what? How are we going to tell him we hated his stupid subject?"
"We didn't hate it!" said Hermione.