I asked the Messenger of Allah ﷺ about a man praying while seated. He said, "Whoever prays standing, that is best. Whoever prays seated receives half the reward of the one who prays standing, and whoever prays lying down receives half the reward of the one who prays seated."
سألت رسول الله ﷺ عن صلاة الرجل قاعدا فقال: إن صلى قائما فهو أفضل، ومن صلى قاعدا فله نصف أجر القائم، ومن صلى نائما فله نصف أجر القاعد.
Bukhari narrated it in the chapter on shortening the prayer (1117, 1116, 1115) through a route from Husayn Muallim, from Abdullah bin Burayda, from Imran bin Husayn, and he related it. Bukhari said, "Lying down beside me here."In another version, "If you cannot, then on your side."His saying, "If he prays standing, that is better," refers to voluntary prayer, because performing the obligatory prayers seated while able to stand is not permissible. His saying, "If you cannot, then on your side" refers to the prayer of a sick person unable to stand, and this involves no reduction in his reward, Allah willing. This is supported by what is established in the Sahih from the hadith of Abu Musa, as a saying of the Prophet, "When the servant falls ill or travels, the same is recorded for him as when he was well and settled." Bukhari narrated it in the chapter on jihad (2996).As for his saying, "And if he prays lying down," they said he prays on his back with his feet toward the qibla, and this is the view of the people of reasoned opinion. Others held that he lies on his right side facing the qibla, and this is the view of Shafii, and it is what the apparent meaning of the Quran and the sunnah indicates. Allah Most High said, "Those who remember Allah standing, sitting, and lying on their sides" [Al Imran 3:191]. See Sharh al-Sunnah (4/112).