Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: Whoever of Allah's creation greets you with salam, return his greeting, even if he is a Magian, for Allah Almighty has said: "When you are greeted with a greeting, respond with one better than it or (one like it)."
Imam Abu Ya'la recorded it, and its narrators are the narrators of the Sahih, except for Ishaq bin Abi Isra'il, and he is thiqah (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 12789