Musnad Ahmad 975:1

Surayj and ‘Affan narrated to us, they said: Abu ‘Awanah narrated to us, Al-Hurr ibn al-Sayyah narrated to us, he said: Surayj [narrated] from Al-Hurr, from Hunaydah ibn Khalid, from his wife, from some of the wives of the Prophet, may God bless him and grant him peace, she said: "The Messenger of God, may God bless him and grant him peace, used to fast the nine [days] of Dhu al-Hijjah, the day of ‘Ashura’, and three days of every month." ‘Affan said: "The first Monday of the month and two Thursdays."