Opening the verse
تَتَجَافَىٰ جُنُوبُهُمْ عَنِ ٱلْمَضَاجِعِ يَدْعُونَ رَبَّهُمْ خَوْفًا وَطَمَعًا وَمِمَّا رَزَقْنَـٰهُمْ يُنفِقُونَ
Their sides part [i.e., they arise] from [their] beds; they supplicate their Lord in fear and aspiration, and from what We have provided them, they spend.
Anas b. Malik said (explaining the meaning of the Qur'anic verse "Who forsake their beds to cry unto their Lord in fear and hope, and spend of what We have bestowed on them" (32:16). The people used to remain awake between the sunset and the night prayers and would pray. Al-Hasan used to say:(This verse means) the prayer and vigil at night
Anas bin Malik said about this Ayah:Their sides forsake their beds (32:16) - "It was revealed about waiting for [this] Salat which you call Al-'Atamah