Zayd bin Wahb (may Allah have mercy on him) narrated: We were travelling with Sayyiduna Ali (may Allah be pleased with him) as he was returning from Siffin, until, when he reached the gate of Kufah, we saw seven graves to our right. Sayyiduna Ali (may Allah be pleased with him) asked: "What are these graves?" The people said: O Commander of the Believers! Sayyiduna Khabbab bin al-Aratt (may Allah be pleased with him) died after you left for Siffin, and he left instructions that he be buried on the outskirts behind Kufah. People used to bury their dead in their courtyards and at the doors of their houses; when the people saw that Khabbab had instructed to be buried on the outskirts, the people also began to bury there. Sayyiduna Ali (may Allah be pleased with him) said: "May Allah have mercy on Khabbab! He accepted Islam willingly, emigrated obediently, lived as a mujahid, and endured trials in his body in many states, and Allah never lets the reward of one who does good deeds go to waste." Then he drew near the graves and said: "Peace be upon you, O people of these abodes, from among the believers and the Muslims! You are our forerunners who have gone ahead, and we shall soon follow after you. O Allah, forgive us and them, and by Your pardon overlook (the faults of) us and them. Blessed is the one who remembered the Hereafter, worked for the Reckoning, was content with sufficiency, and was pleased with Allah, the Mighty and Majestic."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3605