Husn-e-Zann (which is a desired quality as mentioned in the Hadeeth:[Entertain good thoughts about believers.]) requires no proof. Not having any proof for soo-e-zann (evil thoughts) (which is abhorred and prohibited by Allah Ta`ala in the Qur’aan:   (O you who believe! Abstain from suspicions.) [12:49])  and a person merely being a Muslim are sufficient for entertaining good thoughts for a person.

However, one requires proof to have evil thoughts for the next person. (It is sinful to entertain evil thoughts regarding someone without any reliable proof. Allah Ta`ala states:  ’ [Some thoughts are sinful.]

Malfoozaat of Faqeeh-ul-Ummat