Prayer: Purifying the Carpets or Mats of a Mosque
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 891
If the ḥaṣīr[1] or carpet of a mosque becomes impure, it must be washed; and if cutting out the impure part is better, it must be cut out. However, cutting out a considerable amount, or making it pure by causing damage to it, is problematic [i.e. based on obligatory precaution, it must not be done], unless leaving it causes disrespect.
-Ayatullah Sistani, Practical Laws of Islam, Prayer (Salah)
1
A ḥaṣīr is a mat that is made by plaiting or weaving straw, reed, or similar materials of plant origin.