Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2552
'A'isha reported: If one amongst us had to break fasts (of Ramadan due to natural reasons, i. e. menses) during the life of the Messenger of Allah (may peace be upon him) she could not find it possible to complete them so long she had been in the presence of Allah's Messenger (may peace be upon him) till Sha'ban commenced.