Hadith: If any of you wakes up from his sleep and wants to perform ablution, let him blow from his nose the water that he inhales three times, for the devil spends the night on his nostril

عن أبي هريرة رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «إذا اسْتَيقظَ أحدُكم من منامه فتوضأَ فليَستنثرْ ثلاثا، فإن الشيطان يبيت على خَيشُومه».
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "If any of you wakes up from his sleep and wants to perform ablution, let him blow from his nose the water that he inhales three times, for the devil spends the night on his nostril."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

Abu Hurayrah (may Allah be pleased with him) informs us that the Prophet (may Allah's peace and blessings be upon him) said: "If any of you woke up from his sleep and performed ablution", meaning: if he wanted to make ablution. "Let him blow from his nose the water that he inhales three times", meaning: let him wash the inside of his nose thrice (by sniffing water then blowing it out). The prophetic justification of why one should do this if one wakes up from a night's sleep is: "for the devil spends the night on his nostril", meaning: since the devil is not able to whisper to man while sleeping because he is unconscious in his sleep, he spends the night at the tip of his nostril so he can cast bad dreams into his brain and prevent him from having good dreams, as dreams are formed in the brain. Hence, the Prophet (may Allah's peace and blessings be upon him) ordered that one should wash the inside of his nose to remove the devil's filth and stink therefrom. The devil's sleep at night at one's nostril is real because the nose is one of the doors to the heart, and it has no gate just like the ears. Another Hadīth states: "The devil does not open anything closed", and this justifies the order to cover one's mouth when yawning so that the devil would not enter one's mouth.