Abu Dawud Book 7, Hadith Number 2446.
Chapter : Not known.
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.