Hadith: He tells us to worship Allah alone and not associate anything with Him and to give up all what our ancestors said. He also commands us to perform prayers, speak the truth...

عن أبي سفيان صخر بن حرب رضي الله عنه قال: قال هِرَقل: فماذا يَأمُرُكُم -يعني: النبي صلى الله عليه وسلم- قال أبو سفيان: قلت: يقول: «اعبدُوا الله وَحدَه لاَ تُشرِكُوا بِهِ شَيئًا، وَاترُكُوا ما يَقُول آبَاؤُكُم، ويَأمُرُنَا بِالصَّلاَة، والصِّدق، والعَفَاف، والصِّلَة».
[صحيح] - [متفق عليه]

Abu Sufyān Sakhr ibn Harb (may Allah be pleased with him) reported: Heraclius said: "What does the Prophet enjoin you to do?" I said: "He tells us to worship Allah alone, not to associate anything with Him and to give up all that our ancestors said. He also commands us to perform prayers, adhere to truthfulness, be chaste, and maintain ties of kinship."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

This is the famous Hadīth of Abu Sufyān Sakhr ibn Harb (may Allah be pleased with him) about the story of Heraclius. At that time, Abu Sufyān was a polytheist, as he entered Islam later during the period between the Hudaybiyah Peace Treaty and the Conquest of Makkah. Abu Sufyān and a group of people from the Quraysh went to the Levant to meet Heraclius, who was the king of the Christians at that time. He had read the Torah and the Gospel, and knew about the previous holy scriptures. He was an intelligent ruler. When he heard that Abu Sufyān and his companions came to visit him from Hijāz, he summoned them. He started to ask them about the Prophet (may Allah's peace and blessings be upon him) and about his lineage, his Companions, their veneration of him, and his faithfulness. Every time Heraclius asked about something, they told him about it. So he came to know that he was the Prophet which the previous books had mentioned, but as he was keen to retain his kingdom, he did not enter Islam for some reason that Allah, the Almighty, willed. Heraclius asked Abu Sufyān about what the Prophet (may Allah's peace and blessings be upon him) enjoined them to do. Abu Sufyān told him that he ordered them to worship Allah alone and not to associate partners with him. That is, they should worship none but Allah, neither an angel, nor a messenger, or a tree, or a rock, or a sun, or a moon, or any other thing. Worshiping Allah alone is the call of all the messengers. So what the Prophet (may Allah's peace and blessings be upon him) came with was the same call that all other prophets before him came with. He (may Allah's peace and blessings be upon him) ordered them to "give up what your forefathers were upon"; this is a form of proclaiming the truth. The Prophet (may Allah's peace and blessings be upon him) commanded them to abandon the worship of idols which their fathers used to engage in, but he did not order them to give up the noble manners which their forefathers adopted. Abu Sufyān further said: "He ordered us to pray." Prayer is the connection between the slave and His Lord. It is the most emphasized pillar of Islam after the Two Testimonies of faith. Prayer is what distinguishes the believer from the disbeliever. It is the covenant between us (Muslims) and the disbelievers and the polytheists, as the Prophet (may Allah's peace and blessings be upon him) said: "The covenant between us and them is the prayer; whoever abandons it has disbelieved." His statement: "He used to order us to adhere to truthfulness." The Prophet (may Allah's peace and blessings be upon him) used to order his nation to adhere to truthfulness. This is in accordance with Allah’s statement (which means): {O you who believe, fear Allah and be with the truthful} [At-Tawbah: 119]. Truthfulness is a virtuous moral, and it is of two categories: Truthfulness with Allah, and truthfulness with Allah's slaves. Both are from the noble morals. His statement: "chastity". Chastity is of two types: abstention from sexual desire, and abstention from the desires of the stomach. As for the first type: It is when a person abstains from everything that Allah has deemed impermissible for him, like unlawful sexual intercourse, along with all the means and causes related to it. The second type: To abstain from and control the desires of one’s stomach, meaning from what is in the possession of others, and abstain from asking them in such a way that one does not ask anyone for anything, because asking others is humiliation, and the one who asks has the lower hand, while the one who gives has the upper hand. So it is not permissible to ask anyone except for things that are necessarily needed. As for the fifth point: "to maintain ties of kinship". It means that the person joins such relatives which Allah orders him to join, starting from the closest, and then to the closer among them. The closest of them are the parents. Indeed maintaining good relations with the parents constitutes righteousness and maintaining ties of kinship. Ties with the relatives should be maintained in accordance with their closeness. For example, maintaining ties with the brother is more emphasized than the uncle, and maintaining ties with the uncle is more emphasized than the father's uncle. Maintaining ties of kinship can be achieved by all the means that the people are familiar with in this regard.