Answered by Shaykh Amjad Rasheed
Does praying with blood invalidate the prayer (e.g. blood from shaving or blood on one’s body or clothing as a result of killing a fly)?
A. If the blood is from one’s own body, it is excused even if it is a lot unless it is from an orifice (such as the mouth or nose), in which case only a little is excused. The meaning of “a lot” and “a little” is established through what is commonly acknowledged as such (Ar. `urf). Blood that spreads because of sweat or affects one’s garments is also excused because of the hardship [m: involved in avoiding it]. The blood of animals whose blood does not flow when they are killed or when one of their limbs is severed (such as flies, mosquitoes, and the like) takes the same ruling as one’s own blood: both small and great amounts of it are excused, provided that it is not due to one’s own doing. Otherwise, only a small amount is excused.
If the blood that falls upon someone praying is (1) not from himself, and (2) not from the aforementioned case of animals without flowing blood, then only a little is excused.
[m: To summarize, in order for a lot of blood to be excusable, the following conditions have to exist:
1. the blood has to be one’s own, or that of something that doesn’t have flowing blood, and
2. the blood cannot come from an orifice, and
3. the blood cannot be the result of one’s own doing.
In all other cases, only a little blood is excusable.]
Amjad Rasheed
Amman, Jordan
(Translated by Moustafa Elqabbany and Hamza Karamali)