I never saw the Messenger of Allah ﷺ fasting during the ten days at all.
حدثنا أبو معاوية ويعلى قالا حدثنا الأعمش عن إبراهيم عن الأسود
ما رأيت رسول الله ﷺ صائما في العشر قط
Its isnad is sahih. It was previously mentioned at , and what is meant is the ten days of Dhul-Hijjah.