Hadith 1165

Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "When any one of you sleeps, Satan ties three knots at the back of his neck. He recites this incantation at every knot: 'You have a long night, so sleep.' If he awakes and remembers Allah, one knot is loosened. If he performs Wudu', the (second) knot is loosened; and if he performs prayer, (all) knots are loosened. He begins his morning in a happy and refreshed mood; otherwise, he gets up in bad spirits and sluggish state."

[Al-Bukhari and Muslim].