Abu Dawud Book 2, Hadith Number 1301.

Chapter : Prayer and vigil at night.

Narated By Abu Hurairah : The Apostle of Allah (PBUH) said: When one of you sleeps, 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, another knot will be loosened; and if he prays, the third knot will be loosened; and in the morning he will be active and in good spirits; otherwise he will be in bad spirits and sluggish.