Musnad Abu Ya'la al-Mawsili 149:61

Ahmad ibn Isa al-Masri narrated to us, Abdullah ibn Wahb narrated to us, Amr ibn al-Harith informed me, from Ubayd Allah ibn Abi Ja'far, from Muhammad ibn Ja'far ibn al-Zubayr, from Urwah, from Aisha, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever dies while he owes fasting, his guardian shall fast on his behalf."