“He who busies himself with the faults of others will have no time to address his own.”

Abu Hamid al-Ghazali (450–505 AH / 1058–1111 CE) devotes a chapter of the Ihya’ to exactly this problem — how a person can come to know his own faults, in the book on disciplining the self. His answer there is that the fault-finding eye is a finite resource: turned outward it never turns back. This English sentence is the idea in summary; a matching passage in his own words has not been pinned down.