I never saw the Prophet ﷺ fasting during the ten days at all.
حدثنا هناد حدثنا أبو معاوية عن الأعمش عن إبراهيم عن الأسود
ما رأيت النبي ﷺ صائما في العشر قط
Muslim also recorded it in the chapter on itikaf, section on fasting the first ten days of Dhul-Hijjah, hadith no. 9/1176, from the hadith of Abu Muawiya Dareer, from him. The mention of Waki regarding Amash, its chain is sahih.