“This is your copy of Advanced Potion-Making, is it, Potter?'
'Yes,' said Harry, still breathing hard.
'You’re quite sure of that, are you, Potter?'
'Yes,' said Harry, with a touch more defiance.
'This is the copy of Advanced Potion-Making that you purchased from Flourish and Blotts?'
'Yes,' said Harry firmly.
'Then why,' asked Snape, 'does it have the name "Roonil Wazlib" written inside the front cover?'
Harry’s heart missed a beat.
'That’s my nickname,' he said.”