According to Shia jurisprudence, Sajda should be performed on earth, and on those things which are not edible nor worn, and on things which grow from earth (e.g. wood and leaves of trees). The Holy Prophet (pbuh) also performed Sajda on soil and other things which are considered to be a part of earth such as sand, clay, stone, plants (wood) etc. not on carpet, rug, leather and plastic materials.  The Turbah of Imam Hussein (a.s.) on which Sajda is valid and which has superiority over other soils and things, is one of the manifestations of earth (or soil). Hence, Sajda is allowed on it.
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