Musnad Abu Ya'la al-Mawsili 209:20

Abu Bakr ibn Abi Shaybah narrated to us, Abd al-Rahim narrated to us, from al-Hasan ibn Ubayd Allah, from al-Hurr ibn al-Sayyah, from Hunaydah al-Khuza'i, from his wife, from Umm Salamah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said to us: "Fast three days of every month from the beginning of it: Monday, Thursday, and the Thursday that follows it."