بِمُصَيْطِرٍ

بِمُصَيْطِرٍ 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
  
keeper/guardبِمُصَيْطِرٍ=مُصَيْطِرٍ+بِ
wardenwarden / keeperwith
Surah/Below is the only verse in the holy Quran containing the genitive particle and noun compoundبِمُصَيْطِرٍ
Verse
88-22You are not a warden over themلَسْتَ عَلَيْهِم بِمُصَيْطِرٍ