Musnad Abu Ya'la al-Mawsili 117:113

Harun ibn Abdullah narrated to us, Abu Dawud narrated to us, from al-Hakam, from Thabit, from Anas, who said: The Messenger of Allah (peace be upon him) used to go out to the mosque, and the Emigrants (Muhajirun) and the Helpers (Ansar) were in it. None of them would raise his head from his sitting position (hubsah) except Abu Bakr and Umar, for he would smile at them, and they would smile at him.