Prayer: Discovering the Nature or Size of Blood After Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 850
If the area of blood on one’s body or clothing is less than the area covered by a dirham, but he does not know that it is a type of blood that is not excusable in prayers and performs prayers, and afterwards he realises that it was a type of blood that is not excusable in prayers, it is not necessary for him to perform the prayer again. Similarly, if he believes that the amount of blood is less than the area covered by a dirham and performs prayers, and afterwards he realises that it was equal to or more than the area covered by a dirham, it is not necessary for him to perform the prayer again.