Abu Dawud
Fasting
Book Number: 007 / Hadith Number: 2446
Narated By Umm Salamah, Ummul Mu'minin : Hunaydah al-Khuza'i reported on the authority of her mother who said: I entered upon Umm Salamah and asked her about fasting. She said: The Apostle of Allah (pbuh) used to command me to fast three days every month beginning with Monday or Thursday.