I was a man afflicted with many illnesses, so I asked the Messenger of Allah ﷺ about praying while seated. He said, "Your prayer while seated counts as half the reward of your prayer while standing, and a man's prayer while lying down counts as half the reward of his prayer while seated."
حدثنا عبد الوهاب الخفاف عن سعيد عن حسين المعلم قال وقد سمعته من حسين عن عبد الله بن بريدة
كنت رجلا ذا أسقام كثيرة فسألت رسول الله ﷺ عن صلاتي قاعدا قال صلاتك قاعدا على النصف من صلاتك قائما وصلاة الرجل مضطجعا على النصف من صلاته قاعدا
No references are available.
Its chain of transmission is sahih. It is found in Bukhari 2/584, no. 1115, in Fath al-Bari, on shortening the prayer, the prayer of one who is sitting, and in Muslim 1/507, no. 735, on travelers, the permissibility of voluntary prayer while sitting. Also Abu Dawud 1/250, no. 952, and Tirmidhi 2/207, no. 371, who said, hasan sahih, and Nasai 3/223, no. 1660, and Ibn Majah 1/388, no. 1231.