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.” (Using translation from Ibn Mājah 1231)
أَنَّهُ سَأَلَ رَسُولَ اللهِ ﷺ عَنْ صَلَاةِ الْقَاعِدِ فَقَالَ مَنْ صَلَّى قَائِمًا فَهُوَ أَفْضَلُ وَمَنْ صَلَّى قَاعِدًا فَلَهُ نِصْفُ أَجْرِ الْقَائِمِ وَمَنْ صَلَّى نَائِمًا فَلَهُ نِصْفُ أَجْرِ الْقَاعِدِ