Holy Quran, Chapter 42 Verse 21

 وَلَوْلَا كَلِمَةُ الْفَصْلِ لَقُضِيَ بَيْنَهُمْ وَإِنَّ الظَّالِمِينَ لَهُمْ عَذَابٌ أَلِيمٌ

“Had it not been for the decisive word (giving them respite), the decision would certainly have been made between them.  Verily there is a painful chastisement for the unjust”

Imam Baqir (as) said, The words of Allah, “Had it not been for the decisive word (giving them respite), the decision would certainly have been made between them. Verily there is a painful chastisement for the unjust”, mean that if it had not been for the order of Allah, the Qaem (as) would not have left any of (“the unjust”) alive. (Al Kafi volume 8 page 287)

This also means that because of the order of Allah, Imam Mahdi (as) will not destroy “the unjust” until the time of his rising.

• Read More: Mahdi in the Quran