Hadith: I asked the Prophet (may Allah's peace and blessings be upon him): "Which sin is the gravest in the sight of Allah?" He said: "To set an equal to Allah while He created you

عَنْ عَبْدِ اللهِ بنِ مَسْعُودٍ رضي الله عنه قال:
سَأَلْتُ النَّبِيَّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: أَيُّ الذَّنْبِ أَعْظَمُ عِنْدَ اللهِ؟ قَالَ: «أَنْ تَجْعَلَ لِلهِ نِدًّا وَهُوَ خَلَقَكَ» قُلْتُ: إِنَّ ذَلِكَ لَعَظِيمٌ، قُلْتُ: ثُمَّ أَيُّ؟ قَالَ: «وَأَنْ تَقْتُلَ وَلَدَكَ؛ تَخَافُ أَنْ يَطْعَمَ مَعَكَ» قُلْتُ: ثُمَّ أَيُّ؟ قَالَ: «أَنْ تُزَانِيَ حَلِيلَةَ جَارِكَ».
[صحيح] - [متفق عليه] - [صحيح البخاري: 4477]

‘Abdullah ibn Mas‘ūd (may Allah be pleased with him) reported:
I asked the Prophet (may Allah's peace and blessings be upon him): "Which sin is the gravest in the sight of Allah?" He said: "To set an equal to Allah while He created you." I said: "This is, indeed, grave." I said: "What comes next?" He said: "To you kill your child lest he should share your food with you." I said: "What comes next?" He said: "To commit adultery with your neighbor's wife."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 4477]

Explanation

The Prophet (may Allah's peace and blessings be upon him) was asked about the gravest sins and he answered: The gravest sin is major Shirk (polytheism), which means setting up an equal or a rival to Allah in His worship, Lordship, or His names and attributes. Allah Almighty does not forgive this sin except through repentance. If one dies in this state of major Shirk, he will dwell in Hellfire forever. The second gravest sin is killing one's child out of fear that he may share his food with him. So, killing a soul is unlawful, but the sin becomes graver when the killed person is a relative of the killer, and the gravity of the sin increases even more if the killer's motive is fear that the killed person would share with him the sustenance provided by Allah. The third gravest sin is committing adultery with his neighbor's wife by trying to deceive her, so he can have sexual intercourse with her and make her submit to him. Adultery is unlawful but the sin becomes graver when committed with the wife of one's neighbor, whom one is ordered by Shariah to treat with kindness, benevolence, and good companionship.