Abu Dawud Book 2, Number 1301:


Narrated Abu Hurairah:

The Apostle of Allah (peace_be_upon_him) said: When one of you sleeps, the devil ties three knots at the back of his neck, seeling 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.