What is incumbent upon a man who washed his deceased mother after her two-year illness, and his washing her was out of dutifulness to her?

During the time of the Prophet, peace and blessings be upon him, and the Rightly Guided Caliphs, may Allah be pleased with them, it was the established practice that women would wash a deceased woman, with the exception of her husband, for whom it was permissible. Similarly, men would wash a deceased man, with the exception of his wife, for whom it was permissible. Therefore, your washing of your mother is contrary to Islamic law, and you must seek Allah's forgiveness and repent, and not repeat such an action with any of your mahrams in the future.