عَن صَلَاتِهِم

عَن صَلَاتِهِم is a phrase ( شِبْهُ الجُمْلَةِ ) a preposition and a possessed, genitive noun compound

PhraseParticle / Noun / NounNounNoun Particle
شِبْهُ الجُمْلَةِجَارٌ وَمَجْرُوْرٌ وَهُوَ مُضَافٌ وَ مُضَافٌ إِلَيْهِمُضَافٌ إِلَيْهِاسْمٌ مَجْرُوْرٌ وَهُوَ مَضَافٌحَرْفُ جَرٍّ
PhraseCompoundPossessive NounGenitive noun + Noun possessedParticle of pulling down
  
about their prayersعَن صَلَاتِهِم=هِم+صَلَاتِ+عَن
about the prayer of theirstheirsthe prayer ofabout
Surah/Below is the only verse in the holy Quran containing the compoundعَن صَلَاتِهِم
Verse
107-5those who, they are heedless about their prayersالَّذِيْنَ هُمْ عَنْ صَلَاتِهِم سَاهُوْنَ