Musnad Ahmad 1156:10

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 Prophet, peace and blessings of Allah be upon him, used to command me to fast three days of every month: the first of them being Monday, Friday, and Thursday."