Al-Sha'bi, may Allah have mercy on him, said: "If a person, at the time of his death, acknowledges the dowry (mahr) of his wife, this acknowledgment is not valid, except if he had made this acknowledgment while in good health, because she is an heir, and it is not permissible to make a bequest in favor of an heir."
Hadith Referenceسنن سعید بن منصور / كتاب ولاية العصبة / 1502
Hadith Takhrij«أخرجه سعيد بن منصور فى «سننه» ترقيم الدرالسلفية برقم:، 325، وابن أبى شيبة فى «مصنفه» برقم: 17815»