The Chapter on Obligatory Prayers
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 30
وَقَالَ اَلصَّادِقُ عَلَيْهِ اَلسَّلاَمُ: «اَلسُّجُودُ عَلَى اَلْأَرْضِ فَرِيضَةٌ وَ عَلَى غَيْرِ اَلْأَرْضِ سُنَّةٌ.
Imam Jafar ibn Muhammad Al-Sadiq عليه السلام said: "Prostration (Sujood) on the earth is an obligation, and on anything other than the earth is a recommended practice (sunnah)”.
- Ṣaḥīḥ al-Faqīh