📖 Hunaydah Ibn Khalid RadhiAllahu 'anhu narrated: Some of the wives of the Prophet Muhammad SallAllahu 'alaihi wa sallam said that he would fast the first nine days of the month of Dhul-Hijjah, the day of Ashura, and three days of every month, which would be the first Monday and Thursday of the month.
Source: Sunan Abī Dāwūd 2437
Grade: Sahih (Authentic) according to Al-Albani
Allah the Almighty knows best.
Al-Nawawi said, "It is part of the Sunnah (for those Muslims not performing Hajj/pilgrimage) to fast the first nine days of the month of Dhul-Hijjah."
Source: Al-Majmū’ Sharh Al-Muhadhab 6/386
📖 Abu Qatadah RadhiAllahu 'anhu narrated: The Messenger Muhammad SallAllahu 'alaihi wa sallam was asked about fasting the day of 'Arafah (Yawm 'Arafah, the ninth day of Dhul-Hijjah) and he SallAllahu 'alaihi wa sallam said, "It will expiate the sins of the previous year and the upcoming year." Then, the Prophet Muhammad SallAllahu 'alaihi wa sallam was asked about fasting the day of 'Ashura (10th of Muharram) and he SallAllahu 'alaihi wa sallam said, "It will expiate the sins of the past year."
Source: Sahih Muslim 1162
Grade: Sahih (Authentic) according to Muslim