ما هو الدعاء المأثور الذي إذا دعا به الشخص لوالديه كل ليلة يصلهما الثواب؟

There is no reported supplication of the nature you described. However, the supplication of a righteous child benefits their parents, for the Prophet, peace and blessings be upon him, said: "When a person dies, all his deeds cease except for three," and he mentioned among them: "a righteous child who supplicates for him." Among what reaches parents after their death are: praying for them, seeking forgiveness for them, fulfilling their pledges, maintaining kinship ties that are only maintained through them, and honoring their friends.