Mukhtasar Muslim (Abridged) 627 — Fasting three days each month

It is narrated from Sayyida Mu'adhah al-'Adawiyyah that she asked Umm al-Mu'mineen Aisha Siddiqa (may Allah be pleased with her) whether the Messenger of Allah (peace and blessings be upon him) used to fast three days every month. She replied, "Yes." Then she asked, "On which days did he fast?" She said, "He did not mind; he would fast on any day."