once
إذَا سَلَّمَ عَلَيْكُمْ أَهْلُ الْكِتَابِ فَقُولُوا: وَعَلَيْكُمْ
Meaning
When the people of the Book greet you, reply by saying: (And upon you.)
Source and references
Hisn al-Muslim, entry 227.The reference is copied from the edition; separate editorial verification is not complete yet.
Source edition: Fortress of the Muslim. Open source ↗