Musnad Abu Ya'la al-Mawsili 150:77

Ja'far narrated to us, 'Abd al-Warith narrated to us, from Yazid al-Rishk, from Mu'adhah, that a woman asked 'A'ishah: "Did the Messenger of Allah (may Allah bless him and grant him peace) use to fast three days of every month?" She said: "Yes." I said: "Which days of the month did he use to fast?" She said: "He did not mind from which days of the month he fasted."