Sahih al-Bukhari 2976

It is related from Abu Hurayra, "The Messenger of Allah, may Allah bless him and grant him peace, said, 'Shaytan ties three knots at the back of your heads when you are asleep. He hits every knot with the words, 'You have a long night ahead of you, so sleep tight.' When you wake up and remember Allah, one knot is undone. When you do wudu', one knot is undone. When you pray, one knot is undone. So morning finds you cheerful and energetic. Otherwise, morning finds you lazy and foul-tempered.'"