Musnad Ahmad 32:1550

Hussain bin Muhammad narrated to us, Hammad bin Zaid narrated to us, from Salam al-Alawi, from Anas, who said: When the verse of the Hijab was revealed, I came to enter as I used to enter. The Messenger of Allah, peace and blessings of Allah be upon him, said to me: "Stay behind, O my son."