How to respond to the Salam of a third person:

Aishah (radi Allahu anha) said, The Messenger of Allah (sal Allahu alaihi wa sallam) said to me,

"This is Jibril (Gabriel) who is conveying you greetings of peace.”

I responded: "Wa ‘Alaihis-Salamu wa Rahmatullahi wa Barakatuhu.”

[Sahih Al-Bukhari and Muslim]