Since donating blood can save 6 lives per blood donation, if you donate blood with the intention to save 6 lives will you get the reward of saving 6 lives from Allah? Saving one life is like "saving the entire world."
Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Dear brother, you should know that the medical treatment should be done by permissible means in Islam. It is known that using shed blood is forbidden as mentioned in The Qur'an, Allah Says (Interpretation of meaning): {"He has forbidden you only the Maytatah (dead animals), and blood, and the flesh of swine,"}[2:173].
He also Says: {"Forbidden to you (for food) are: Al-Maytatah (the dead animals - cattle-beast not slaughtered), blood,"}[5:3].
But if a person is compelled to use the blood of a person, and there is no other way for his treatment, then using it is permissible provided that medical authorities confirm that this is the only way for this treatment. Because Allah Says: after declaring it forbidden {But if one is forced by necessity without will disobedience nor transgressing due limits, then there is no sin on him.}[2:173].
He also Says: {"while He has explained to you in detail what is forbidden to you, except under compulsion of necessity? And surely many do lead (mankind) astray by their own desires through lack of knowledge. Certainly your Lord knows best the transgressors."}[6:119].
Therefore, one can donate his blood if this does not cause him any harm. It is permissible also to make this request from a Muslim or some one else for the purpose of saving the lives of compelled people be they Muslims or non-Muslims who are not in a state of war with Muslims.
Therefore, whoever donates his blood to save the lives of people, indeed, he gets a good reward for that if he does so for the sake of Allah. Allah Says (interpretation of meaning): {Whoever brings a good deed (Islâmic Monotheism and deeds of obedience to Allâh and His Messenger SAW) shall have ten times the like thereof to his credit, and whoever brings an evil deed (polytheism, disbelief, hypocrisy, and deeds of disobedience to Allâh and His Messenger SAW) shall have only the recompense of the like thereof, and they will not be wronged.}[6:160].
Allah also Says: {So whosoever does good equal to the weight of an atom (or a small ant), shall see it. And whosoever does evil equal to the weight of an atom (or a small ant), shall see it.}[99:7-8].
No one knows the amount of reward Allah will grant him for this act.
Allah knows best.
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