It was narrated from ‘Imran bin Husain that he asked the Messenger of Allah (ﷺ) about a man who prays sitting down. He said, “Whoever performs prayer standing up, that is better. Whoever performs prayer sitting down will have half the reward of one who prays standing. And whoever performs prayer lying down will have half the reward of one who prays sitting.”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا بِشْرُ بْنُ هِلاَلٍ الصَّوَّافُ، حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ، عَنْ حُسَيْنٍ الْمُعَلِّمِ، عَنْ عَبْدِ اللَّهِ بْنِ بُرَيْدَةَ، عَنْ عِمْرَانَ بْنِ حُصَيْنٍ، أَنَّهُ سَأَلَ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ عَنِ الرَّجُلِ يُصَلِّي قَاعِدًا قَالَ " مَنْ صَلَّى قَائِمًا فَهُوَ أَفْضَلُ. وَمَنْ صَلَّى قَاعِدًا فَلَهُ نِصْفُ أَجْرِ الْقَائِمِ. وَمَنْ صَلَّى نَائِمًا فَلَهُ نِصْفُ أَجْرِ الْقَاعِدِ " .
sunnah.com