وَمَنْ أَصَابَهُ مِنَ النَّجَاسَةِ الْمُغَلَّظَةِ، كَالدَّمِ وَالْبَوْلِ وَالْغَائِطِ وَالْخَمْرِ، مِقْدَارَ الدِّرْهَمِ وَمَا دُونَهُ: جَازَتِ الصَّلَاةُ مَعَهُ، وَإِنْ زَادَ: لَمْ تَجُزْ.
Translation: If a person is affected by severe impurity such as blood, urine, feces or wine in an amount no greater than a dirham, prayer is permissible; more than that invalidates prayer.
Excused quantity does not mean that the filth has turned pure. The substance remains impure, while prayer validity is considered with a defined tolerance. The legal measure of a dirham is not simply the weight of a modern coin.