بُلُوغُ الْمَرَامِ
Bulugh al-Maram 328
The Book of Prayer
]وَعَنْ عِمْرَانَ بْنِ حُصَيْنٍ -رَضِيَ اَللَّهُ عَنْهُمَا- قَالَ : { قَالَ لِيَ اَلنَّبِيُّ - صلى الله عليه وسلم -" صَلِّ قَائِمًا , فَإِنْ لَمْ تَسْتَطِعْ فَقَاعِدًا , فَإِنْ لَمْ تَسْتَطِعْ فَعَلَى جَنْبٍ } رَوَاهُ اَلْبُخَارِيُّ . [1]
Narrated 'Imran bin Husain (RA): Allah's Messenger (ﷺ) said: "Pray standing and if you are unable, pray sitting and if you cannot, pray lying on your side, [otherwise pray by signs]." [Reported by al-Bukhari, (without the final words)].
Sunnah.com reference: Book 2, Hadith 223