الَّذِينَ هُمْ عَن صَلَاتِهِمْ سَاهُونَ
those who are heedless of their prayers. (107:5)
Hum is the subject of the nominal construction and sāhūn its predicate; ʿan ṣalātihim is a prepositional expression associated with the predicate and placed before it. The preposition ʿan directs the description to heedlessness concerning prayer, not automatically to a passing distraction occurring within prayer.
The fronted complement makes the neglected duty salient. Syntax, preposition, and context must be considered together.