Al-Matalib al-'Aliyah 2528 — The rights of a Muslim over a Muslim

It is narrated from Imam Hasan al-Basri (may Allah have mercy on him) that he said: "They (the Companions and the predecessors) used to say: Indeed, a Muslim is the mirror of his Muslim brother; he shows him those (conditions or faults) of his that he himself cannot see in his own self."
Imam Ahmad bin Hanbal (may Allah have mercy on him) narrated it in his book al-Zuhd.