He saw the Prophet ﷺ perform ablution, and he wiped his head with fresh water, not with the leftover moisture from his hands.
حدثنا علي بن خشرم أخبرنا عبد الله بن وهب حدثنا عمرو بن الحارث عن حبان بن واسع عن أبيه
أنه رأى النبي ﷺ توضأ وأنه مسح رأسه بماء غير فضل يديه
Muslim also narrated it in the Book of Tuhur, Chapter on Another Description of Wudu, hadith no. 236, from the hadith of Ibn Wahb, with it being the hadith of Ibn Lahia [Ahmad narrated it: 4/39–41, and Darimi: 1/180, with the wording, "with water other than the leftover from his hands"].