In Unity, GameObjects derived from prefabs don't inherently keep a reference to those prefabs. Furthermore, to prevent serialization issues, when a prefab with a reference to itself is instantiated, the new object gets a reference to itself -- not the original prefab.
This repository provides a few scripts that allow developers to easily create and maintain stable references to source prefabs.
Copy RememberPrefab.cs to your scripts folder and RememberPrefabCustomEditor.cs to a folder named Editor.
Add RememberPrefab as a component to the desired object and select the appropriate prefab reference in the inspector. Note: Due to limitations on how Unity loads files in runtime, RememberPrefab can only store references to prefabs inside of a folder named Resources. This folder can be anywhere, there can be multiple such folders, and it can have its own subfolders. But there needs to be a folder named "Resources" somewhere in the path for this component to work. See https://docs.unity3d.com/ScriptReference/Resources.html for more information.
Call GetComponent<RememberPrefab>().Prefab to get a reference to the prefab in code -- this works in either Edit or Play mode, and the reference always points to the same object no matter what.
Not guaranteed to work correctly when values of RememberPrefab are modified via script instead of the inspector, particularly during builds.