Musannaf Ibn Abi Shayba 33:289

Abu Usama narrated to us, he said: Abdullah bin Muhammad bin Umar bin Ali narrated to me, he said: My father narrated to me, he said: It was said to Ali, "What is your affair, O Abu al-Hasan, that you have taken the graveyard as a neighbor?" He said, "Indeed, I find them to be neighbors of truth; they refrain from evil and remind [one] of the Hereafter."