What are the legitimate proofs, Quranic verses, and fatwas that can alleviate my mother's fear of death, taking into account that she raised her children after their father's death and did not remarry for their sake?

It is recommended to ask about the mother's well-being and to take turns sitting with her, reminding her of the virtues of righteous deeds, and that whoever seeks his reward from Allah does not fear death. Advise her to ask Allah for a long life, and remind her of the hadith (definition): "The best of people is he whose life is long and whose deeds are good, and the worst of people is he whose life is long and whose deeds are evil." Remembering death is a good thing.