Sahih Al Bukhari – Book of Virtues Of The Qur'an

Volumn 006, Book 061, Hadith Number 535.
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Narated By 'Aisha RA: Whenever Allah's Apostle PBUH became sick, he would recite Mu'awwidhat (Surat Al-Falaq and Surat An-Nas) and then blow his breath over his body. When he became seriously ill, I used to recite (these two Suras) and rub his hands over his body hoping for its blessings.