فِي القُبُوْرِ

فِي القُبُوْرِ is a phrase (شِبْهُ الجُمْلَةِ) consisting of a particle of pulling down (preposition) and a genitive noun

PhraseParticle / NounNoun Particle
شِبْهُ الجُمْلَةِجَارٌ وَمَجْرُوْرٌاسْمٌ مَجْرُوْرٌحَرْفُ جَرٍّ
PhraseCompoundGenitive NounParticle of pulling down
  
in the gravesفِي القُبُوْرِ=القُبُوْرِ+فَي 
in the gravesthe gravesin
Surah/Below are the 3 verses in the holy Quran containing the compoundفِي القُبُوْرِ
Verse
22-7
35-22
100-4Knows he not that when the contents of the graves are poured forthأَفَلَا يَعْلَمُ إِذَا بُعْثِرَ مَا فِي الْقُبُوْرِ