My question is about the permissibility in Islam for a Muslim to report a theft to the authorities or the affected company. The person has witnessed stealing of money from that company. And also the sinner admitted his sin before him but he is not mending his ways. Is this a duty on the Muslim to report this sin if he thinks that the person is going to steal more in the future or at least not return the stolen money back?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Islam encourages covering people's faults and not spreading them among the public. The Prophet said: "Whosoever covers the (sins of) a Muslim, Allaah covers (his sins) on the Day of Judgment." [Al–Bukhari]

Similarly, Islam encourages enjoining good, forbidding evil, and preventing sinners from wickedness. The Prophet said: "Whoever sees an evil, should change it with his hand; if he can't, then he should change it with his tongue; if he can't then he should reject it in his heart (i.e. detest it); and it is the weakest faith." [Muslim]

Jareer Bin Abdullaah said: "I have given a pledge of allegiance to Allaah's Apostle to offer prayers perfectly, to pay Zakat, and to offer sincere advice to every Muslim." [Al–Bukhari]

The above narration includes giving advice for a non-Muslim too. Imaam Ibn Hajar said: 'Even though the narration limits offering advice to a Muslim since it is the common practice, yet giving good advice to a non-Muslim is also recommended…'

Therefore, if one finds someone stealing, he should prevent him from that evil and should order him to return the stolen items to their legal owner without disclosing his mistakes in order to cover him. If he does not take heed and does not return what was stolen, then you have to inform the owner to fulfill your duty of offering sincere advice. It becomes more emphatic if your silence may cause an innocent person to be accused of stealing.

Allaah Knows best.