Musnad Abu Ya'la al-Mawsili 117:215

Ahmad ibn al-Dawraqi narrated to us, Abu Dawud narrated to us, al-Hakam ibn Atiyyah al-Ayshi narrated to us, Thabit narrated from Anas, who said: The Messenger of Allah (peace be upon him) used to go out to the mosque where the Emigrants (Muhajirun) and the Helpers (Ansar) were. There was not one among them who would raise his head from his sitting posture (habwah) except for Abu Bakr and Umar. He would smile at them, and they would smile at him.