Turn off the Instant Previews feature

By default, the Instant Previews feature is turned on.
  1. On the home screen, swipe down from the top of the screen.
  2. Tap Settings > Notifications.
  3. Set the Instant Previews switch to Off.
Note: You can display only an application's priority messages as Instant Previews. On the Notifications Settings screen, tap the application and in the Instant Previews drop-down list tap Priority Only.