Sahih Al Bukhari – Book of Prayers (Salat)

Volumn 001, Book 008, Hadith Number 464.
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Narated By 'Abbad bin Tamim : That his uncle said, "I saw Allah's Apostle lying flat (on his back) in the mosque with one leg on the other." Narrated Said bin Al-Musaiyab that 'Umar and 'Uthman used to do the same.