How to find the (original) Prefab of a GameObject?

Getting access to a prefab, from an instantiated object is an editor only functionality - I.E. it will only work at edit time, not run time.

There are other solutions to your issue though.

Don't destroy the objects. If you disable them instead, you can renable them later

 void DisableGameObject(GameObject objectToDisable)
 {
     objectToDisable.SetActive(false);
 }
 
 void EnableGameObject(GameObject objectToEnable)
 {
     objectToEnabled.SetActive(true)
 }

Firstly sorry for my english. If you want you can add a property of the original prefab on the instance in a script. For Example you can create a script with a enumerator

using UnityEngine;

using System.Collections;

public class PrefabInstance : MonoBehaviour {

 public enum PrefabProperty {FirstPrefab, SecondPrefab, ThirdPrefab};
 public PrefabProperty prefabProperty;

}

and add it to the instance

using UnityEngine; using System.Collections;

public class InstantiatePrefab : MonoBehaviour {

 public GameObject prefab;
 public List<PrefabInstance> instances = new List<PrefabInstance>();

 // Use this for initialization
 void InstantiateSomePrefabs () {
     GameObject obj = GameObject.Instantiate(prefab) as GameObject;

     PrefabInstance instance = obj.AddComponent<PrefabInstance>();
     instance.prefabProperty = PrefabInstance.PrefabProperty.FirstPrefab;
     instances.Add(instance);
 }

}

for search all game objects whit the same property

using System.Linq;

 public PrefabInstance[] InstancesFromFirstPrefab(){
     PrefabInstance[] tmpInstances = instances.Where(d => d.prefabProperty == PrefabInstance.PrefabProperty.FirstPrefab).ToArray();
     return tmpInstances;
 }