Musnad Ahmad 26:770

Mu'adh ibn Mu'adh narrated to us, he said: Sufyan ibn Sa'id narrated to us, from 'Abdullah ibn al-Sa'ib, from Zadhan, from 'Abdullah ibn Mas'ud, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, Allah, the Almighty and Majestic, has angels who travel about the earth; they convey to me the greeting (salam) from my Ummah."