Musnad Ahmad 26:627

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Mansur, who said: I heard Abu Wa'il narrating from Abdullah, who said: We used to say: "Peace be upon so-and-so and so-and-so." Then the Messenger of Allah (peace and blessings of Allah be upon him) said: "Say: 'The greetings (at-tahiyyat) are for Allah, and the prayers and the good things. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us, and upon the righteous servants of Allah.' For if you say: 'Peace be upon us, and upon the righteous servants of Allah,' you have sent peace upon every righteous servant in the earth and in the heaven."