Sahih Al Bukhari – Book of Ablutions (Wudu')
Volumn 001, Book 004, Hadith Number 155.
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Narated By Abu Qatada : Allah's Apostle PBUH said, "Whenever anyone of you drinks water, he should not breathe in the drinking utensil, and whenever anyone of you goes to a lavatory, he should neither touch his p*n*s nor clean his private parts with his right hand."