By default, the Instant Previews feature is
On the home screen, swipe down from the top of the screen.
Settings > Notifications.
Set the Instant Previews switch to Off.
Tip: For some apps, you can change your notification settings to only
display the priority messages in an app as Instant Previews. On the
Notifications settings screen, tap an application. In the Instant Previews drop-down list, tap Priority Only.