Sahih Al Bukhari - Book of Shortening The Prayers (At-Taqseer)
Volumn 002, Book 020, Hadith Number 216.
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Narated By 'Imran bin Husain : (Who had piles) I asked Allah's Apostle about the praying of a man while sitting. He said, "If he prays while standing it is better and he who prays while sitting gets half the reward of that who prays standing; and whoever prays while Lying gets half the reward of that who prays while sitting."
