Maliks Muwatta Book 18, Hadith Number 54.
Section : Making Up For Days Not Fasted in Ramadan in General.
Yahya related to me from Malik from Yahya ibn Said from Abu Salama ibn Abd ar-Rahman that he heard A’isha, the wife of the Prophet, may Allah bless him and grant him peace, say, “I used to have to make up days from Ramadan and not be able to do them until Shaban came.”