فِي جَنَّةٍ

فِي جَنَّةٍ 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 a gardenفِي جَنَّةٍ=جَنَّةٍ+فِي
in a paradisea paradisein
Surah/Below are the 2 verses in the holy Quran containing the preposition and genitive nounفِي جَنَّةٍ
Verse
1
69-22in a lofty paradiseفِي جَنَّةٍ عَالِيَةٍ
88-10in a lofty paradiseفِي جَنَّةٍ عَالِيَةٍ