I never saw the Messenger of Allah (ﷺ) fasting in the ten days of Dhu'I-Hijja.
Sahih Muslim Book 14, Hadith 10
← Previous Hadith 'A'isha (Allah be pleased with her) reported that Allah's Messenger (ﷺ) used to exert himself in devotion during the last ten nights to a greater extent than at any other time.

Sahih Muslim Book 14, Hadith 9 Next Hadith → The Apostle of Allah (ﷺ) did not observe fast in the ten days of Dhul-Hijja.

Sahih Muslim Book 14, Hadith 11