Sahih Al Bukhari - Book of Shortening The Prayers (At-Taqseer)
Volumn 002, Book 020, Hadith Number 218.
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Narated By 'Imran bin Husain : Had piles, so I asked the Prophet about the prayer. He said, "Pray while standing and if you can't, pray while sitting and if you cannot do even that, then pray Lying on your side."
