Sahih Muslim Book 6, Hadith Number 2549.

Chapter : Permissibility of completing the fasts of Ramadhan before the commencement of the coming Ramadhan.

Abu Salama reported: I heard ‘Aisha (Allah be pleased with her) as saying: I had to complete some of the fasts of Ramadan, but I could not do it but during the month of Sha’ban due to my duties to the Messenger of Allah (may peace be upon him) or with the Messenger of Allah (may peace be upon him).