I have heard that bleeding invalidates the fast. This could be from a minor cut from a knife or from a severe accident.
What is the exact ruling about this?
What about minor bleeding caused by popping of a pimple, nose bleeding for no obvious reasons or blood in the stool?
If bleeding did invalidate fast, our beloved Prophet (Peace and mercy of Allah be upon him) would never undergo cupping while fasting.
Sahih Al-Bukhari
Book 30 (Fasting) :: Hadith 45
Narrated Ibn `Abbas:The Prophet (pbuh) was cupped while he was in the state of lhram, and also while he was observing a fast.
Book 30 (Fasting) :: Hadith 46
Narrated Ibn `Abbas:The Prophet (pbuh) was cupped while he was fasting.
Book 76 (Medicine) :: Hadith 16
Narrated Ibn `Abbas:The Prophet (pbuh) was cupped while he was fasting.
Cupping/Hijama is a process that sucks blood through a small incision made in a person's skin.
If you don't know what cupping is, go through this page from Wikipedia.
However, Hanbali school debates that Cupping invalidates the fast, NOT due to the flow of blood, but because of the temporary weakness it offers. This is albeit a different question you could ask.
One thing you should obviously be cautious is that when nasal or oral bleeding occurs, maximum care should be taken not to let the blood down the throat.
What invalidates the fast is bloodletting or cupping. This is because of the following narration from the Prophet (salallahu alayhi wassalam)
Shaddad bin Aus (Radiallahu anhu) narrated that The Messenger of Allah (salallahu alayhi wassalam) came across a man in Al-Baqi' (in Madinah) who was having himself cupped in Ramadan., and said to him, “The one who cups and the one who is being cupped have both broken their fast." Related by the five Imams except for At-Tirmidhi. Ahmad, Ibn Khuzaimah and Ibn Hibban regarded it as authentic.
Scholars use the same hadeeth to conclude that intentional blood leaving like donating blood or treatment by letting blood out will also invalidate the fast.
However, blood exiting from a fasting person unintentionally does not break the fast because he is excused for that. Examples being nosebleed or the blood from surgery or a tooth extraction.
Lessons in Fasting - Shaykh Salih Al-Fawzaan
On the side note, there is disagreement on this issue. Majority of the scholars view cupping in Ramadhaan as permissible because of other Hadeeths that state that the Prophet sallallaahu `alayhi wa sallam was cupped while he was fasting. You can read a detailed treatment of the issue here.
