The reason why Shi`as prostrate on the turba is based on two sound premises hinging upon two issues:
FIRST: It is commendable for one who prays to use a pure soil he is sure to be clean regardless of where he takes it from; there is no preference nor any distinction of one soil over another in as far as prostrating upon it is concerned. This is only an indication that the person offering his prayers demonstrates concern about the cleanliness of his body and clothes, and about the place where he offers his prayers. It is something which a Muslim regards as a good plateau upon which he prostrates when at home or when travelling, one which he may not find everywhere, i.e. other towns, bazaars, hotels, inns, rest areas, parks, traveller lodges, transportation stations, ports, shelters, etc. How can one do so since such places may be frequented by non-Muslims and other people who are heedless of and indifferent to religion when it comes to a spot which by necessity has to be pure and clean?
What is wrong with a Muslim taking a precaution relevant to his creed by carrying with him a pure soil with which he feels comfortable and of the cleanliness of which he is sure to prostate upon when he offers his prayers rather than prostrating on filth, uncleanness and impurities? The latter never bring one closer to Allah, nor does the Sunnah permit prostrating upon them, nor doing so is accepted by sound reason once a person has made sure that his body and clothes are clean. Moreover, the Sunnah prohibits offering prayers in places such as: garbage collection sites, slaughter-houses, cemeteries, highways, toilets, places where camels rest, mandating that mosques should be cleansed and perfumed .