The Questioner: What is the ruling of the blood of a human or the blood of an animal if it falls upon clothing, does it become impure (najis)? And what is the ruling on [someone] bleeding during prayer (ṣalāh), does it invalidate the prayer or not?
Shaykh Muqbil: If the blood comes from a person and it does not spill in the Masjid, then there is no harm for him to continue his prayer. It is the correct view that it does not nullify wuḍūʾ (ablution) and it is not impure (نجس – najis). Nothing is made impure from blood, except the blood of which is impermissible to eat from, and the blood from menstruation (الحيض – al-ḥayḍ) and postnatal bleeding (النفاس- an-nifās). As for whatever else of blood, then there is no proof of its impurity.
Instead, ʿAbdāllah ibn ʿUmar, may Allāh be pleased with him, used to come forward when he had a butrah—meaning, some plants had broken off onto him, and he would crush them [which caused bleeding], and then thereafter he still prayed [without remaking his wuḍūʾ]. And the Companions of the Prophet ﷺ used to pray while they were wounded.
Ghārat al-Ashriṭah (1/67–68)