Musnad Ahmad 32:1435

Yunus and Mu'ammal narrated to us, they said: Hammad—meaning Ibn Zayd—narrated to us, Salam al-'Alawi narrated to us, Anas ibn Malik narrated to us, he said: When the verse of the hijab was revealed, I went to enter as I used to enter, so the Prophet, peace and blessings of Allah be upon him, said to me: "Behind you, O my son."