34. Narrated Abu Huraira (radhi Allaahu anhu) : Allah’s Messenger (Sallallaahu alaihi wa sallam) said,
“When one of you wakes up from his sleep, he must clean his nose (with water, by sniffling it in and then blowing it out) three times, for Satan spends the night inside one’s nostrils.”  [Agreed upon].
 Satan’s spending the night inside the nostrils of a man is plausible. However, its actual state is better known to Allah. It may be that this is a figurative expression, because bad excretions gather in the nose and cause laziness, indolence, slackness and negligence. And all these states are satanic dispositions. Blowing water out of the nose upon awakening from sleep might be performed as an individual act, or along with ablution. It is considered obligatory by Ahmad, while the majority of scholars consider it Sunnah.
Bulugh al-Maram – Collection of hadith by al-Hafidh ibn Hajar al-Asqalani. With brief notes from the book Subul-us-Salam written by: Muhammad bin Ismail Al-Sanani.
English Publication- Book 1, Hadith 34. Darus-salam Publishers.