Question
Where is it written, “You see in others what you see in the mirror.”?
Answer
I believe what you are referring to is the Talmudic passage “Kol HaPosel B’moomo Posel” – “Anyone who disqualifies others, disqualifies them with his own flaw (Kiddushin 70b). Often when one renders someone else unfit/imperfect (e.g. having shortcomings) it is indicative that the one passing judgment himself possesses those same flaws.
Many Blessings,
Rabbi Zvi Solomon