Musnad Ahmad 30:1757

Ishaq ibn Isa narrated to us, Malik informed us, from al-Ala, from his father, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) went out to the graveyards and said: "Peace be upon you, O abode of a believing people, and we, if Allah wills, will be joining you."