Musnad Ahmad 1153:1109

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Yazid al-Rishk, from Mu‘adhah, from ‘Aishah, that she said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to fast three days of every month." She said: So I said: "From which [part] of it?" She replied: "He did not care from which [part] of it it was."