Sunnah.com reference : Book 2, Hadith 358English translation : Book 2, Hadith 439Arabic reference : Book 2, Hadith 443
Narrated Jabir (RA):
The Prophet (ﷺ) visited a sick man and saw him praying on a cushion, he threw it away and said, "Pray on the ground, if you can, or else pray by gestures and make your prostration lower than your bowing."

[al-Baihaqi reported it and Abu Hatim graded it Sahih (authentic) in Mawquf form (as a saying of a companion, i.e. Jabir)].
وَعَنْ جَابِرٍ قَالَ: { عَادَ اَلنَّبِيُّ ‏- صلى الله عليه وسلم ‏-مَرِيضًا, فَرَآهُ يُصَلِّي عَلَى وِسَادَةٍ, فَرَمَى بِهَا, وَقَالَ: 
"صَلِّ عَلَى اَلْأَرْضِ إِنْ اِسْتَطَعْتَ, وَإِلَّا فَأَوْمِ إِيمَاءً, وَاجْعَلْ سُجُودَكَ أَخْفَضَ مِنْ رُكُوعِكَ" }  رَوَاهُ اَلْبَيْهَقِيُّ.‏ وَصَحَّحَ أَبُو حَاتِمٍ وَقْفَهُ.‏