Firebase Firestore detach a listener doesn't work

I (think I) am following the instructions on how to detach a listener in Firebase Firestore, but in my code the listener remains active.

// attaching a listener
this.dbTasks.where('completed', '==', true).onSnapshot(data => {...code...})

// detaching a listener
this.dbTasks.where('completed', '==', true).onSnapshot(() => {})

What am I missing?