What Can and Cannot Be Used for Prostration
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 41
وَ رُوِيَ عَنْ يَاسِرٍ اَلْخَادِمِ أَنَّهُ قَالَ: مَرَّ بِي أَبُو اَلْحَسَنِ عَلَيْهِ اَلسَّلاَمُ وَ أَنَا أُصَلِّي عَلَى اَلطَّبَرِيِّ وَ قَدْ أَلْقَيْتُ عَلَيْهِ شَيْئاً فَقَالَ لِي «مَا لَكَ لاَ تَسْجُدُ عَلَيْهِ أَ لَيْسَ هُوَ مِنْ نَبَاتِ اَلْأَرْضِ.
It was narrated from the Yāsir al-Khadim that He said: "Abu al-Hasan عليه السلام passed by me while I was praying on a piece of ṭabarī (a mat) upon which I had placed something. Imam عليه السلام said to me: 'Why do you not prostrate on it? Is it not from what the earth produces?'"
- Ṣaḥīḥ al-Faqīh, p. 41