Hadith.828 - Imam Jafar ibn Muhammad Al-Sadiq (as) said:
"Prostration (Sujood) on the earth is obligatory (fardh), while prostrating on other than that is a recommended practice (sunnah)."
828 - قال الصادق عليه السلام: " السجود على الأرض فريضة وعلى غير ذلك سنة.
Similar narrations
Al-Kāfi #1251“Abu ’Abd Allah (a.s.), has said, ’Performing Sujud (prostration) on earth is obligatory and on a prayer mat is a tradition.”