Musnad Ahmad 1155:37

Hashim ibn al-Qasim narrated to us, Abu Ishaq al-Ashja'i al-Kufi narrated to us, he said: 'Amr ibn Qays al-Mula'i narrated to us, from al-Hurr ibn al-Sayyah, from Hunaydah ibn Khalid al-Khuza'i, from Hafsah, she said: "Four things the Prophet, peace and blessings of Allah be upon him, would not abandon: fasting Ashura, the ten (days of Dhu al-Hijjah), three days of every month, and the two rak'ahs before the dawn prayer."