What is the ruling of Islamic law concerning one who gave a heart patient medicine that he thought was beneficial, but it caused the patient's death due to his ignorance of the medicine, and is his act considered a religious crime, and how can he expiate his sin?

An-Nasa'i and Abu Dawud reported that the Messenger of Allah (peace be upon him) said: "Whoever practices medicine without prior knowledge of it is a guarantor (of any harm he causes)." This means that whoever practices medicine without prior knowledge or experience is liable for the consequences of his actions.

If it is medically proven that the substance caused the death, then the one who administered it must fast for two consecutive months as expiation for accidental killing, and he must pay the blood money (diyah) to the heirs of the deceased. The 'Aqilah (male relatives on the father's side) do not bear this blood money.

However, if it is not medically proven that the substance caused the death, then no expiation or blood money is required. The one who administered it must repent to Allah for his action, as Allah the Almighty says: "And do not pursue that of which you have no knowledge." (Al-Isra: 36)