Musnad Ahmad 26:155

Waki’ narrated to us, Sufyan narrated to us, from Jabir, from Abu al-Duha, from Masruq, from Abdullah, who said: Among the things I have not forgotten is that the Messenger of Allah (peace and blessings of Allah be upon him) used to say the taslim to his right and to his left: "As-salamu alaykum wa rahmatullah, As-salamu alaykum wa rahmatullah," until the whiteness of his cheeks could be seen, or we could see the whiteness of his cheeks. Waki’ narrated to us, from Sufyan, from Abdullah ibn al-Sa’ib, from Zadan, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Verily, Allah has angels who travel about the earth, conveying to me the greeting (salam) from my Ummah."