Sahih Muslim Book 4, Hadith Number 1702.

Chapter : Preference for prolonging recitation in the night prayer.

Abu Huraira transmitted it from the Apostle of Allah (may peace be upon him): 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.