Hadith 31-91

وَبِهذَا الإِسنادِ قالَ: قالَ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَآلِهِ: إِذا لَمْ يَسْتَطِعِ الرَّجُلُ أَن يُصَلّيَ قائِماً فَليُصَلَّ جالِساً فَإِنْ لَمْ يَقدِر أَن يُصَلّيَ جالِساً فَليُصَلَّ مُستَلقِياً، ناصِباً رِجلَيهِ بِحِيالِ القِبْلَةَ يُومي إيماءً.

According to the same documentation, God’s Prophet (S) said, “Whenever a believer cannot pray standing up, he should say his prayers while sitting down. If he cannot say them sitting down, he can say them while lying on his back with his legs stretched out toward the Qibla.”