Musnad Ahmad 32:426

Abu Kamil Muzaffar ibn Mudrik narrated to us, Hammad ibn Zayd narrated to us, from Silm al-Alawi, who said: I heard Anas ibn Malik saying: When the verse of the hijab was revealed, I came to enter as I used to enter, and the Prophet, peace and blessings of Allah be upon him, said: "Stay behind, O my son."