Is it permissible to use impure material for Istijmar?
Chapter on Cleaning oneself (Istitabah) and Impurity (Hadath)
Al-Mughni
Book of Purification
Primary text
The material used for Istijmar must be pure. If it is impure, it is not sufficient. This is the view of Al-Shafi'i. The rationale is established by the action of the Prophet (peace be upon him) who took two stones brought by Ibn Mas'ud but discarded a piece of dung (rawtha), declaring it to be 'rijs' (filth), which serves as an explicit legal reason. Purification from filth cannot be achieved by another filth, similar to washing with impure water.
Supporting text
Abu Hanifa permits the use of impure material, arguing that it dries the impurity and cleanses the site, thus being equivalent to a stone. There is also a secondary consideration: if impure material is used, it is debated whether subsequent water purification is necessary, as the site may have become defiled by an external impurity, requiring water, or whether it is sufficient as the impurity is secondary to the original site impurity.