What is the ruling on praying for one's parents to die to relieve them of the severe suffering caused by illness?

It is not permissible to wish for death due to harm, such as illness or hardship; rather, patience and seeking reward from Allah are obligatory. For Allah grants relief and a way out for the sick person, and He rewards him for his patience. Allah Almighty says: "Indeed, he who fears Allah and is patient, then verily, Allah wastes not the reward of the good-doers" and "And do not despair of the mercy of Allah. Indeed, no one despairs of the mercy of Allah except the disbelieving people." The Prophet, peace and blessings be upon him, forbade wishing for death due to harm. But if one must do so, then let him say: "O Allah, keep me alive as long as life is better for me, and cause me to die if death is better for me." Wishing for death unconditionally is an objection to destiny and despair of Allah's mercy. Moreover, illness may contain good, by expiating sins and increasing reward. The reported supplication is an entrustment and submission to divine decree, and it is permissible in times of complaint and distress, although patience and submission are more preferable and bring greater reward.