Yahya related to me from Malik from Ibn Shihab that A'isha and Hafsa, the wives of the Prophet, may Allah bless him and grant him peace, began fasting voluntarily one morning and then food was given to them and they broke their fast with it. Then the Messenger of Allah, may Allah bless him and grant him peace, came in. A'isha said, "Hafsa asked, anticipating me in speech - she took after her father Umar - 'Messenger of Allah, A'isha and I began the morning fasting voluntarily and then food was given us and we broke the fast with it.' The Messenger of Allah, may Allah bless him and grant him peace, said, 'Fast another day in its place.' "
USC-MSA web (English) reference: Book 18, Hadith 50
Arabic reference: Book 18, Hadith 683