Musnad Ahmad 1017:104

Muhammad ibn Humayd Abu Sufyan narrated to us, from Sufyan, from Alqamah ibn Marthad, from Sulayman ibn Buraydah, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to teach them when they went out to the graveyards to say: "Peace be upon you, O people of the dwellings, from among the believers and the Muslims. And we, if Allah wills, will surely join you. You are our predecessors, and we are your followers. We ask Allah for well-being for us and for you."