This is a scene from Harry Potter and the Half-Blood Prince, page 171.
Snape: Do you remember me telling you we are practising non-verbal spells, Potter?
Snape: Yes sir.
Harry: There's no need to call me "sir," Professor.
From left to right: Hermione Granger, Harry Potter, Ron Weasley, Lavender Brown, and Parvati Patil.