Musnad Abu Ya'la al-Mawsili 150:257

Ahmad ibn Isa 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 having [obligatory] fasting upon him, his guardian shall fast on his behalf."