Is a person sinful for having ill thoughts about those who work alongside him, by saying: "Perhaps they are so, and they intend so," and is it obligatory for him to seek their forgiveness, and is ill-thought like backbiting in the obligation to seek forgiveness?

It is forbidden to harbor ill thoughts about a Muslim from whom no suspicion has appeared, for indeed, assumption is the falsest of speech. The forbidden assumption occurs when it settles in the heart, as opposed to fleeting thoughts. Whoever falls into forbidden ill assumption must repent to Allah. As for seeking forgiveness from the one about whom ill thoughts were harbored, it is not prescribed, because informing him may cause mischief. Rather, repentance between him and Allah, seeking forgiveness for him, and speaking well of him suffice.