Using Turbah in Salat
Why do Shi'a prostrate on turbah?
Shi'a consider the use of turbah to be a practice of the Sunna (tradition) of the Prophet, who used to put his forehead on the soil during prostration. According to hadiths, the Prophet (s) forbade a person from prostrating on a carpet and approved a person who brought a stone to the mosque and prostrated on it. Lady Fatimat al-Zahra' (s) made a turbah from the soil of the grave of Hamzah, the uncle of the Prophet (s). After the martyrdom of Imam al-Husayn (a), Imam al-Sajjad (a) made a turbah from the soil of Karbala and prostrated on it during his salats.
Prostration on Soil in Tradition of the Prophet and Conduct of His Sahaba
'Abd al-Wahhab al-Shafi'i, the Sunni scholars: “I have not found in the authentic Sunni sources, a reliable narration that explicitly permits prostration on carpets or clothes, but rather I have found that these are not permitted.”
Shi'a and Sunni have narrated many hadiths indicating that the Prophet (s) used to put his forehead on the soil for prostration. According to hadiths, the Prophet (s) said that the earth had been made a purifier and a place of prostration for him. Based on a hadith mentioned in the book of Kanz al-'ummal, the Prophet (s) forbade a person who prayed on the carpet (prostrated on the carpet) from doing so and removed the carpet. 'Abd Allah b. 'Umar, the son of the second Caliph, has narrated that he went to the mosque for morning prayer in a rainy night. [While he was there, he saw] a person picked a suitable stone for prayer, spread a cloth and placed the stone on it and prayed. The Prophet (s) saw him and considered his action beautiful and good.
Based on these hadiths, Shi'a scholars believe that prostration should be performed on the soil or something that is allowed to be prostrated on in the Sunna of the Prophet (s).
following the Prophet (s), Muslims used to prostrate on the soil. Some hadiths narrate that Abu Bakr, the first caliph, always prayed on the ground, and if he stood on a carpet, he would put his forehead on the soil. He also forbade a person from prostrating on a cloth. 'Umar, the second caliph believed that prostration is placing the forehead and face on the earth and he prostrated on the ground. 'Uthman, the third caliph, also prostrated on the ground. 'Abd Allah b. Masoud, one of the Sahaba of the Prophet (s), and one of the narrators of the early hadith in Islam, also prayed only on the ground. When some Muslims went on a trip, they made a turbah from the soil of Medina and took it with them to prostrate on it.
Using Turbah by Ahl al-Bayt
After the martyrdom of Hamza b. 'Abd al-Muttalib, the uncle of the Prophet (s) in the battle of Uhud, Lady Fatima al-Zahra' (s) made a rosary and a turbah from the soil of Hamza's grave.
Imam al-Sajjad (a) also made a rosary from the soil of Karbala after the martyrdom of Imam al-Husayn (a). Following their Imams, Shi'a used the turbah of Karbala until it became a slogan and a tradition for Shi'a around the world.