What is the ruling on treating cancer solely with ruqyah (exorcism according to Islamic law) while refusing medical treatment, on the pretext that the disease is from jinn?

There is no contradiction between belief in God and reliance upon Him, and seeking medical treatment and visiting a doctor. Healing comes from God, and the Prophet, peace be upon him, commanded and encouraged seeking treatment, saying: "Indeed, Allah Almighty, where He created the ailment, He also created the cure, so seek treatment," and "Seek treatment, for Allah has not created an ailment without creating a cure for it." Similarly, seeking medical treatment does not contradict seeking healing through the Quran, ruqyah (definition) (incantation), and supplication. The Quran is a cure for ailments of the heart and body, and legitimate ruqyah is beneficial. The servant combining reliance on God, supplication, and taking means, such as visiting doctors, is the essence of true reliance, as expounded by Ibn al-Qayyim, for neglecting means impairs and weakens reliance.