Mukhtasar Muslim

Abu Salamah says: I heard from Umm al-Mu'mineen Aisha Siddiqa (may Allah be pleased with her) that she said: "Whenever I had to make up missed fasts of Ramadan, I could not make them up except in Sha'ban, and the reason for this was that I used to be occupied in the service of the Messenger of Allah (peace and blessings be upon him) and would not find the opportunity."