How do I stop my iPhone from unlocking when I slide on a text

That's not something that is possible unless you have previously unlocked the iPhone by using the home button. since it scans your print and unlocks the device.

If you turn it on via the side button, and slide the notification, it will show it to you, but the iPhone should remain locked.

There is no security issue here. Try pressing the home button with a finger you have not registered for Touch Id, and then slide the notification. It will not unlock the iPhone at all.

If you pressed the home button with a registered finger it will unlock at that point.and as such open the app when sliding the notification to view it.

Set a passcode on the phone and turn on Touch-ID. If you want, you can prevent notifications for things like text messages from appearing on the home screen altogether.

On which iPhone?