What Can and Cannot Be Used for Prostration
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 41
وَقَالَ يُونُسُ بْنُ يَعْقُوبَ: رَأَيْتُ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ يُسَوِّي اَلْحَصَى فِي مَوْضِعِ سُجُودِهِ بَيْنَ اَلسَّجْدَتَيْنِ.
Yunus ibn Ya'qub said: "I saw Imam Jafar ibn Muhammad Al-Sadiq عليه السلام leveling the pebbles at the place of his prostration between the two prostrations."
- Ṣaḥīḥ al-Faqīh, p. 41