Musannaf Ibn Abi Shayba 17:383

Ibn ‘Ulayyah narrated to us, from Khalid ibn Ayyub, from Mu’awiyah ibn Qurrah, who said: My father advised me. He said: If he greets you, do not say "wa ‘alayka" (and upon you), but say "wa ‘alaykum" (and upon you all), for there are angels with him.