Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
quran:51:17 –

They used to sleep but little 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,
القرآن:٥١:١٧ –

كَانُوا قَلِيلًا مِنَ اللَّيْلِ مَا يَهْجَعُونَ  

{كانوا قليلا من الليل ما يهجعون} ينامون، وما زائدة ويهجعون خبر كان وقليلا ظرف، أي ينامون في زمن يسير من الليل ويصلون أكثره.