Musnad Abu Ya'la al-Mawsili 209:104

Abu Khaythama narrated to us, Muhammad ibn Fudayl narrated to us, al-Hasan ibn Ubayd Allah narrated to us, from Hunaydah al-Khuza'i, from his mother, who said: I entered upon Umm Salamah and asked her about fasting. She said: The Messenger of Allah (may Allah bless him and grant him peace) used to command me to fast three days of every month: the first of them being Monday, then Thursday, then Monday.