Sahih Bukhari Volume 2, Book 20, Hadith Number 216.

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.”