Sahih Al Bukhari – Book of Bathing (Ghusl)
Volumn 001, Book 005, Hadith Number 285.
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Narated By 'Umar bin Al-Khattab RA : I asked Allah's Apostle PBUH "Can any one of us sleep while he is Junub?" He replied, "Yes, if he performs ablution, he can sleep while he is Junub."