What is the prescribed manner of prayer for one unable to pray sitting?
Chapter on the Times When Prayer is Prohibited
Al-Mughni
Book of Prayer
Primary text
If a person is incapable of praying while sitting, they must pray on their side, turning their face toward the Qibla. This is the position referred to as the sleeping posture. This ruling is held by Malik, Al-Shafi'i, Ibn Al-Mundhir, and is supported by the Hadith stating: "The prayer of one who is lying down is half the prayer of one who is sitting," and the Prophet's explicit instruction: "If he cannot, then on his side."
Supporting text
Saeed ibn Al-Musayyib, Al-Harith Al-Akkli, Abu Thawr, and the Companions of Reason (Ahl al-Ra'y) hold that the sick person should pray lying flat on their back (mustalqiyan), with their face and feet oriented towards the Qibla, to ensure their gesture (iemaa') is directed towards it, unlike lying on the side where the face points away from the Qibla during the gesture.