Musnad Ahmad 32:1233

Rawh narrated to us, Jarir ibn Hazim narrated to us, from Salam al-Alawi, from Anas ibn Malik, who said: I used to serve the Messenger of Allah, may Allah bless him and grant him peace, and I used to enter upon him without permission. Then I came one day and entered upon him, and he said: "O my son, a matter has occurred, so do not enter upon me except with permission."