Abu Dawud Book 36, Hadith Number 4870.

Narated By Mu’awiyah : I heard the Apostle of Allah (PBUH) say: If you search for the faults of the people, you will corrupt them, or will nearly corrupt them. AbudDarda’ said: These are the words which Mu’awiyah himself from the Apostle of Allah (PBUH), and Allah benefited him by them.