The Messenger of Allah forbade us from spying (Al-Tajassus) and that we should not backbite one another; Allah the Almighty said: {...and do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it.} (Al-Hujurat: 12).
The Messenger of Allah—may Allah’s peace and blessings be upon him and his family—forbade us from looking at the faults (‘uyub) of our believing brothers and from being negligent toward them. The Messenger of Allah (PBUH&HP) said: "May Allah curse the one who looks at the nakedness (sawa’ah) of his brother."
He intends by that [the previous quote] the one who looks at the failings of the brothers1.
The Messenger of Allah (PBUH&HP) said:
"Indeed, an immorality (fahiša) is broadcast among the believers until it reaches a believer like him; once it reaches him, it is placed within a secure sanctuary (hirz hariz) and concealed."
Thus, whoever kills, steals, commits adultery, spies, backbites the believers, looks at their women, looks at their faults, or commits any of the prohibitions, vile acts, and immoralities that Allah has forbidden, has indeed opposed Allah and His Messenger. And whoever opposes Allah and His Messenger is not a wise man.