This principle was best expressed that I've heard so far as "It is better to find one fault in yourself, than to find a thousand in someone else.". (The Suite Life. Ep: 2x06 T: 21:42)

Actually I think it is better worded like this : "It is better to find one fault in yourself than many faults in others.".

And an important note is that "yourself" can apply to an individual or a group. This can be very powerful when looked at as "find fault in yourself" refers to your country or other larger group.