Musnad Ahmad 26:660

Waki' and Abd al-Rahman narrated to us, they said: Sufyan narrated to us, from Abdullah ibn al-Sa'ib, from Zadhan, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: (Waki' said): "Indeed, Allah has angels who travel about the earth, conveying to me the peace (salam) from my Ummah."