The Prophet ﷺ used to fast on the day of Ashura, the first nine days of Dhul-Hijjah, and three days each month, the first Monday of the month and two Thursdays.
أخبرني زكريا بن يحيى قال حدثنا شيبان قال حدثنا أبو عوانة عن الحر بن صياح عن هنيدة بن خالد
أن النبي ﷺ كان يصوم يوم عاشوراء وتسعا من ذي الحجة وثلاثة أيام من الشهر أول اثنين من الشهر وخميسين
[Its isnad is sahih] Reported by Abu Dawud, Book of Fasting, chapter on fasting the ten days, no. 2437, from the hadith of Abu Awana Al-Waddah, with this same wording. It is also found in Al-Kubra, no. 2681.Note: Hunaida is a companion, and his wife is also a companion, as mentioned in the section on unnamed women in Taqrib Tahdhib.