Nafi' (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the sick person should pray lying on his back, with his feet towards the qiblah. This report is mawquf (stopping at the Companion).
(b) And this is to be understood as applying when he is unable even to lie on his side. 'Wa billahi al-tawfiq' (And Allah alone grants success).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3679