Musnad Abu Ya'la al-Mawsili 65:12

Ishaq ibn Abi Isra'il narrated to us, Humayd—meaning al-Ru'asi—narrated to us, Hasan ibn Salih narrated to me, from Simak ibn Harb, from 'Ikrimah, from Ibn 'Abbas, who said: "Whoever greets you from the creation of Allah, return the greeting to him, even if he is a Magian (Zoroastrian). For Allah says: 'And when you are greeted with a greeting, greet [in return] with one better than it or [at least] return it' (An-Nisa: 86)."