Musnad Ahmad 32:311

Yazid narrated to us, Hammad al-Tawil informed us, from Anas ibn Malik, who said: Umm Sulaym took my hand upon the arrival of the Prophet, may God bless him and grant him peace, in Medina. She brought me to the Messenger of God, may God bless him and grant him peace, and said: "O Messenger of God, this is my son, and he is a young boy who can write." He said: "So I served him for nine years, and he never once said to me regarding anything I had done: 'Why did you do this?' or 'How evil is what you have done.'"