﴿ كَانُوا۟ قَلِيلًۭا مِّنَ ٱلَّيْلِ مَا يَهْجَعُونَ ﴾

“they would lie asleep during but a small part of the night,”

Little of the night did they use to sleep mā is extra; yahja‘ūna is the predicate of kānū; qalīlan ‘little’ is an adverb in other words they used to sleep for a small portion of the night and perform prayers during most of it