three knots

Sahih Bukhari 2.237: Narrated Hudhaifa : Whenever the Prophet got up for Tahajjud prayer he used to clean his mouth (and teeth) with Siwak.

Sahih Bukhari 2.243: Narrated Abu Huraira Allah's Apostle said, "Satan puts three knots at the back of the head of any of you if he is asleep. On every knot he reads and exhales the following words, 'The night is long, so stay asleep.' When one wakes up and remembers Allah, one knot is undone; and when one performs ablution, the second knot is undone, and when one prays the third knot is undone and one gets up energetic with a good heart in the morning; otherwise one gets up lazy and with a mischievous heart."