Musnad Abu Ya'la al-Mawsili 210:17

Abu Bakr ibn Abi al-Nadr narrated to us, Abu al-Nadr narrated to us, Abu Ishaq al-Ashja‘i—and he is not Ubayd Allah—narrated to us, from ‘Amr ibn Qays, from al-Hurr ibn al-Sayyah, from Hunaydah ibn Khalid al-Khuza‘i, from Hafsah, who said: "There were four things that the Messenger of Allah (may Allah bless him and grant him peace) would never abandon: fasting the ten (days of Dhu al-Hijjah), Ashura, fasting three days of every month, and two rak‘ahs before the dawn prayer."