Musnad Ahmad 32:574

Sulayman ibn Dawud narrated to us, Ibn 'Atiyyah—meaning al-Hakam—narrated to us, from Thabit, from Anas, who said: "The Prophet, peace and blessings of Allah be upon him, would go out to the mosque where the Emigrants (Muhajirun) and the Helpers (Ansar) were. None of them would raise his head from his sitting position (huthwah), except for Abu Bakr and Umar. He would smile at them, and they would smile at him."