عن عمران بن حصين رضي الله عنه قال: كَانَتْ بِي بَوَاسِيرُ، فَسَأَلْتُ النَّبِيَّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ عَنِ الصَّلَاةِ، فَقَالَ: «صَلِّ قَائِمًا، فَإِنْ لَمْ تَسْتَطِعْ فَقَاعِدًا، فَإِنْ لَمْ تَسْتَطِعْ فَعَلَى جَنْبٍ».
The Prophet (may Allah's peace and blessings be upon him) pointed out that the main rule is to pray while standing, except when one is unable to do so, in which case he can pray while sitting, and if he cannot, he can pray while lying on his side.
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