It was narrated that 'Imran bin Husain said: "I asked the Prophet (ﷺ) about one who prays sitting down. He said: 'Whoever prays standing up is better, and one who prays sitting down will have half the reward of one who prays standing up. And whoever prays lying down will have half the reward of one who prays sitting down.'"
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا حُمَيْدُ بْنُ مَسْعَدَةَ، عَنْ سُفْيَانَ بْنِ حَبِيبٍ، عَنْ حُسَيْنٍ الْمُعَلِّمِ، عَنْ عَبْدِ اللَّهِ بْنِ بُرَيْدَةَ، عَنْ عِمْرَانَ بْنِ حُصَيْنٍ، قَالَ سَأَلْتُ النَّبِيَّ صلى الله عليه وسلم عَنِ الَّذِي يُصَلِّي قَاعِدًا قَالَ " مَنْ صَلَّى قَائِمًا فَهُوَ أَفْضَلُ وَمَنْ صَلَّى قَاعِدًا فَلَهُ نِصْفُ أَجْرِ الْقَائِمِ وَمَنْ صَلَّى نَائِمًا فَلَهُ نِصْفُ أَجْرِ الْقَاعِدِ " .
sunnah.com