Musnad Ahmad 1153:584

Hashim narrated to us, Ishaq bin Sa'id narrated to us, from his father, who said: It was said to Aisha: "O Mother of the Believers, this month is twenty-nine [days]." She said: "And what surprises you about that? For I fasted with the Messenger of Allah, peace and blessings of Allah be upon him, twenty-nine [days] more often than I fasted thirty."