Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: We used to say in the prayer: Peace be upon our Lord. Then it was said to us: Say: "Peace be upon us and upon the righteous servants of Allah"; when you say this, you will have sent greetings of peace upon whoever is in the heavens and the earth.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9898