Musnad Ahmed: Hadith 4150

We used to say: Peace be upon So and so and So and so. Then The Messenger of Allah (ﷺ) said: "Say: ‘All compliments, prayers and pure words are due to Allah. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us, and on the righteous slaves of Allah. I bear witness that there is no god but Allah, and I bear witness that Muhammad is His slave and Messenger.’ For when you say, 'Peace be upon us, and on the righteous slaves of Allah,' you will have sent salams upon every righteous slave on earth and in heaven.”