I never saw the Messenger of Allah ﷺ fast the ten days at all. In another wording, the Prophet ﷺ did not fast the ten days.
ما رأيت رسول الله ﷺ صائما العشر قط. وفي لفظ: أن النبي ﷺ لم يصم العشر.
Muslim narrated it in the chapter on itikaf (1176) via Amash, from Ibrahim, from Aswad, from Ayesha, as related above. Tirmidhi also narrated it through this same route (756) and said, "This is how more than one narrator reported it from Amash, from Ibrahim, from Aswad, from Ayesha." Thawri and others narrated this hadith from Mansur, from Ibrahim, that the Prophet ﷺ was never seen fasting at all.As for what has been narrated from some of the wives of the Prophet ﷺ, that the Prophet ﷺ used to fast the first ten days, this is shaz (anomalous), contradicting what is found in the sahih. Nasai narrated it (2418) from the hadith of Hunayda bin Khalid, from his wife, from one of the wives of the Prophet ﷺ.