Yazid bin Abi Yazid, al-Rashk يزيد بن أبي يزيد الضبعي الرشك · Fasting (Kitab Al-Siyam)

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 Messenger of Allah (ﷺ) used to command me to fast three days every month beginning with Monday or Thursday.