Sahih Muslim 776
Sahih Muslim · Book 6: The Book of Prayer - Travellers · Encouragement to pray at night even if it is little
Abu Huraira transmitted it from the Messenger of Allah (ﷺ):
حَدَّثَنَا عَمْرٌو النَّاقِدُ ، وَزُهَيْرُ بْنُ حَرْبٍ ، قَالَ عَمْرٌو حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ ، عَنْ أَبِي الزِّنَادِ ، عَنِ الأَعْرَجِ ، عَنْ أَبِي هُرَيْرَةَ ، يَبْلُغُ بِهِ النَّبِيَّ صلى الله عليه وسلم " يَعْقِدُ الشَّيْطَانُ عَلَى قَافِيَةِ رَأْسِ أَحَدِكُمْ ثَلاَثَ عُقَدٍ إِذَا نَامَ بِكُلِّ عُقْدَةٍ يَضْرِبُ عَلَيْكَ لَيْلاً طَوِيلاً فَإِذَا اسْتَيْقَظَ فَذَكَرَ اللَّهَ انْحَلَّتْ عُقْدَةٌ وَإِذَا تَوَضَّأَ انْحَلَّتْ عُقْدَتَانِ فَإِذَا صَلَّى انْحَلَّتِ الْعُقَدُ فَأَصْبَحَ نَشِيطًا طَيِّبَ النَّفْسِ وَإِلاَّ أَصْبَحَ خَبِيثَ النَّفْسِ كَسْلاَنَ " .
When any one of you goes to sleep, the devil ties three knots at the back of his neck, sealing every knot with:" You have a long night, so sleep." So if one awakes and mentions Allah, a knot will be loosened; if he performs ablution two knots are loosened; and if he prays (all) knots will be loosened, and in the morning he will be active and in good spirits; otherwise we will be in bad spirits and sluggish in the morning.