Musnad Abu Ya'la al-Mawsili 148:26

Abu al-Rabi' al-Zahrani narrated to us, Hammad bin Zayd narrated to us, Salam al-'Alawi narrated to us, he said: I heard Anas bin Malik say: When the verse of the Hijab was revealed, I used to enter as I used to enter. The Messenger of Allah (may Allah bless him and grant him peace) said to me: "Behind you, O my son."