Reference: Fataawa Ramadhaan – Volume 2, Page 465, Fatwa No.401
al-Fataawa libni ‘Uthaymeen – Kitaab ad-Da’wah – Volume 1, Page 175
Question: Does the drawing of a lot of blood render the fast broken?
Response: If the drawing of a lot of blood resulted in weakness of the body and the need for nourishment – similar to that which occurs after cupping, then its ruling is similar to the ruling of cupping. As for that which comes out without the person’s choice, such as when a man is injured and a lot of blood flows out, then this does not affect [the validity of the fast] because this [occurred] without the person intending so.