What Can and Cannot Be Used for Prostration
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 41
وَ رُوِيَ عَنْ عَلِيِّ بْنِ بَجِيلٍ أَنَّهُ قَالَ: رَأَيْتُ جَعْفَرَ بْنَ مُحَمَّدٍ عَلَيْهِ اَلسَّلاَمُ كُلَّمَا سَجَدَ فَرَفَعَ رَأْسَهُ أَخَذَ اَلْحَصَى مِنْ جَبْهَتِهِ فَوَضَعَهُ عَلَى اَلْأَرْضِ.
It was narrated from Ali ibn Bajil that He said: "I saw Imam Jafar ibn Muhammad Al-Sadiq عليه السلام, every time he prostrated and then raised his head, he would take the pebbles from his forehead and place them back on the ground."
- Ṣaḥīḥ al-Faqīh, p. 41