Praying while seated for medical reasons
What is the ruling on praying while seated for medical reasons, since praying while standing may result in forgetting [some of the motions of prayer]?
Prayers are meant to be performed in a manner by which one is aware of the motions of prayer. If a legally responsible Muslim prays while standing but is not aware of his actions except if he prays while seated, then he is included under the words of the Prophet who said, "Pray while standing; if you cannot, pray while seated; if you cannot, pray while lying down."
It is permissible for a Muslim to pray while seated if standing will result in forgetting some of the motions of prayer.
 Narrated by Bukhari through Umran ibn Hussein [may Allah be pleased with him] in a raised chain of transmission.