Musnad Abu Ya'la al-Mawsili 210:10

Abu Bakr ibn Abi Shaybah narrated to us, Hashim ibn al-Qasim narrated to us, al-Ashja'i narrated to us, from 'Amr ibn Qays, from al-Hurr ibn al-Sayyah, from Hunaydah ibn Khalid, from Hafsah, who said: "There were four things that the Messenger of Allah (ﷺ) would never abandon: fasting the day of Ashura, the ten (days of Dhu al-Hijjah), three days of every month, and the two rak'ahs before the dawn prayer."