Musnad Ahmad 937:204

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Zayd ibn Wahb, from Abu Dharr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O Abu Dharr, raise your sight and look at the most elevated man you see in the mosque." He said: So I looked, and behold, there was a man sitting wearing a fine garment. He said: So I said, "This one." He said: Then he said, "O Abu Dharr, raise your sight and look at the most lowly man you see in the mosque." He said: So I looked, and behold, there was a weak man wearing worn-out clothes. He said: So I said, "This one." He said: Then the Messenger of Allah (peace and blessings of Allah be upon him) said: "By the One in Whose hand is my soul, this one is better in the sight of Allah on the Day of Resurrection than a fill of the earth like that one."