Musnad Ahmad 1153:392

Yahya narrated to us, Ibn Lahi'ah informed us, and Musa ibn Dawud said: Ibn Lahi'ah narrated to us, from Ubayd Allah ibn Abi Ja'far, from Muhammad ibn Ja'far ibn al-Zubayr, from Urwah, from Aisha, that she asked the Messenger of Allah, may Allah bless him and grant him peace. And Musa said: The Prophet, may Allah bless him and grant him peace, said: "Whoever dies while he has fasting [obligations] upon him," the Messenger of Allah, may Allah bless him and grant him peace, said: "His guardian shall fast on his behalf."