Rights of worship (rights of slaves)
The major sins which are related to the rights of Allah (Allah's rights), such as failure to pay prayers, fasting, zakat and Hajj, Allah will forgive him for true repentance from Allah, inshaAllah. But if these sins are related to the rights of slaves, such as stealing someone's belongings, or harming someone, or violating someone's rights, then all the scholars and jurists agree on this in the light of the Qur'an and Hadith. In order to forgive him, it is first necessary to pay the right of the servant who has a right over us, or to forgive him, then turn to Allah Almighty for repentance and forgiveness.
The Holy Prophet (peace and blessings of Allaah be upon him) said: All the sins of a martyr are forgiven, but the debt of a person. (Muslim, 2) That is, if a person owes a debt to someone, he will remain in debt until it is paid, no matter how great a good deed is done. The famous narrator Hazrat Imam Nawawi (may Allah be pleased with him) writes in his commentary on this hadith that “debt” means “all rights of worship”. (Sharh Muslim)
It was learned that we should not be negligent in paying our dues. The Holy Prophet (sws) said: Do you know who is poor? The companions replied: In our opinion, a poor person is one who has no money and no worldly possessions. The Prophet (peace and blessings of Allaah be upon him) said: The poorest of my ummah is the one who will bring many prayers, fasting, zakaah (and other popular acts of worship) on the Day of Resurrection, except that he may have insulted someone. If someone has been slandered, someone's wealth has been eaten, someone's blood has been shed or someone has been beaten, then one of his virtues will be given to the one who has the right (according to his right), similar to other rights. He will be rewarded with good deeds out of his good deeds. Then if all his good deeds are taken away before the rights of others are paid, then the sins of the deserving and the oppressed (which they have done in this world) will be taken away from him and passed on to him. And then that person will be thrown into hell. (Muslim - Chapter of Prohibition of Oppression) This is the poverty of this Muslim Ummah that in spite of many good deeds, it will be thrown in Hell due to negligence in the rights of worshipers.
May Allah grant us all the rights of Allah as well as the rights of worshipers
You are right, The Prophet (peace and blessings of Allaah be upon him) said: The poorest of my ummah is the one who will bring many prayers, fasting, zakaah (and other popular acts of worship) on the Day of Resurrection, except that he may have insulted someone.