Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) had a Christian scribe. Once Sayyiduna Abdullah (may Allah be pleased with him) entered the palace and the Christian went back, so Sayyiduna Abdullah (may Allah be pleased with him) sent a greeting of salam after him.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8922