Musnad Ahmad 1153:505

Hashim ibn al-Qasim narrated to us, he said: Ishaq ibn Sa'id narrated to us, from his father, who said: It was said to Aisha: "O Mother of the Believers, this month was sighted at twenty-nine." She said: "And what surprises you about that? For I fasted with the Messenger of Allah, may Allah bless him and grant him peace, twenty-nine [days] more often than I fasted thirty."