He was asked about the Prophet's ﷺ use of dye, and he replied, "He did not reach the point of needing to dye his hair. If I wished, I could count the gray hairs in his beard."
سئل عن خضاب النبي ﷺ فقال: إنه لم يبلغ ما يخضب، لو شئت أن أعد شمطاته في لحيته.
Bukhari narrated it in the Book of Clothing (5895) and Muslim in the Book of Virtues (1341:23) both through Hammad bin Zayd from Thabit, who said, Anas was asked, and he mentioned it. The wording given is that of Bukhari. As for the wording of Muslim, it includes, Anas said, "If I wished to count the gray hairs that were on his head, I could do so." And he said, "He did not dye his hair, though Abu Bakr dyed his with henna and katam."