Sahih Bukhari
Shortening the Prayers (At-Taqseer)
Volume: 002 / Book Number: 020 / Hadith Number: 216
Narrated '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."