Sahih Bukhari
Sahih Bukhari Volume 003, Book 031, Hadith Number 171.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 31. Fasting
Narated By 'Aisha : Sometimes I missed some days of Ramadan, but could not fast in lieu of them except in the month of Sha'ban." Said Yahya, a sub-narrator, "She used to be busy serving the Prophet."