بُلُوغُ الْمَرَامِ
Bulugh al-Maram 329
The Book of Prayer
وَعَنْ جَابِرٍ - رضى الله عنه - { أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -قَالَ لِمَرِيضٍ - صَلَّى عَلَى وِسَادَةٍ , فَرَمَى بِهَا - وَقَالَ : " صَلِّ عَلَى اَلْأَرْضِ إِنْ اِسْتَطَعْتَ , وَإِلَّا فَأَوْمِئْ إِيمَاءً , وَاجْعَلْ سُجُودَكَ أَخْفَضَ مِنْ رُكُوعِكَ } رَوَاهُ اَلْبَيْهَقِيُّ بِسَنَدٍ قَوِيٍّ وَلَكِنْ صَحَّحَ أَبُو حَاتِمٍ وَقْفَهُ . [1]
Narrated Jabir (RA): The saying of the Prophet (ﷺ) to a sick person who prayed on a cushion: The Prophet (ﷺ) threw it away and said, "Pray on the ground, if you are able to do so; otherwise, pray by gesturing signs and make your prostration lower than your bowing." [al-Baihaqi reported it through a strong chain of narrators, but Abu Hatim regarded it as Mauquf (saying of a Companion)].
Sunnah.com reference: Book 2, Hadith 224