Musannaf Ibn Abi Shayba 33:91

Abu Mu’awiyah and Ya’la narrated to us, from al-A’mash, from Zayd ibn Wahb, from Abu Dharr, who said: The Prophet (peace be upon him) said to me: "Look, O Abu Dharr, at the most prominent man you see in the mosque." He said: So I looked, and there was a man wearing a fine garment, so I said: "This one." He said: Then he said: "Look at the most lowly man you see in the mosque." He said: So I looked, and there was a man wearing worn-out clothes, so I said: "This one." He said: Then he said: "This one is better than a fill of the earth of that one."