Is it permissible to file a complaint with official authorities—to prevent corruption—based on the testimony of a watchman who swore that prostitution is being practiced in a number of apartments in his neighbor's house, and is his testimony considered valid in Sharia?

Accusing someone of immorality without proof is one of the gravest sins, and spying on people is forbidden unless it is to prevent an evil when suspicion arises. If suspicion does arise among apartment residents, it is permissible to inform the official authorities, but not merely based on the words of the doorman or his wife. The doorman and his wife must be made aware of the gravity of defaming people without just cause. If they have proof of the existence of an evil, then they should be the ones to inform the official authorities.