Musnad Ahmad 32:1124

Yazid ibn Harun narrated to us, Humayd informed us, from Anas, that Umm Sulaym took his hand upon the arrival of the Messenger of Allah, peace and blessings of Allah be upon him, in Medina. She said: "O Messenger of Allah, this is Anas, my son, and he is a young boy who can write." Anas said: "So I served him for nine years, and he never said to me regarding anything I had done: 'Why did you do this?' or 'How evil is what you have done!'"