It is narrated from Hazrat Aisha (may Allah be pleased with her) that she asked the Prophet (peace and blessings be upon him) about the ruling if a person dies and there are some fasts due upon him. The Prophet (peace and blessings be upon him) said: His guardian should fast on his behalf.
Hadith Referenceمسند احمد / مسند النساء / 24402
Hadith Gradingحکم دارالسلام:حديث صحيح بالرواية السابقة