Hunaydah bin Khalid (may Allah be pleased with him) narrates from his wife, from one of the wives of the Prophet (peace and blessings be upon him) (may Allah be pleased with them), that the Messenger of Allah (peace and blessings be upon him) used to fast the 9th of Dhu al-Hijjah, the day of Ashura, and three days of every month: the first Monday of the month and Thursday.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8393