Musnad Abu Ya'la al-Mawsili 209:11

Muhammad ibn Abdullah ibn Numayr narrated to us, Ibn Fudayl narrated to us, al-Hasan ibn Ubaydullah narrated to us, from Hunaydah ibn Khalid 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 the fasting of three days of every month, from the beginning of them: Monday and Thursday, and a day I do not remember.