Reference: http://saif.af.org.sa/ar/node/2269
Question: What is the ruling regarding the prayer of the person who has a large spot of blood on his clothing?
Response: If he knew of it and prayed whilst knowing of it, then his prayer is incorrect because of the impurity of the blood. If however he did not know [of the spot of blood], or he knew [of it] but forgot – and then prayed having forgotten, then I anticipate there is no harm [in him having done so] in shaa.-Allaah.
