Abdullah ibn Mas’ud reported: I said, “O Messenger of Allah, what is the greatest sin?” The Prophet, peace and blessings be upon him, said, “To set up a partner alongside Allah, even though He alone created you.” I said, “What is next?” The Prophet said, “To kill your child, fearing he will eat with you.” I said, “What is next?” The Prophet said, “To commit adultery with the wife of your neighbor.” Then, Allah confirmed the saying of the Prophet with the verse, “Those who do not call upon another god besides Allah, nor kill a soul Allah has made sacred, except by right of justice, and do not commit adultery.” (25:68)
Source: Ṣaḥīḥ al-Bukhārī 6001, Ṣaḥīḥ Muslim 86
Grade: Muttafaqun Alayhi (authenticity agreed upon) according to Al-Bukhari and Muslim
عَنْ عَبْدِ اللَّهِ قَالَ قُلْتُ يَا رَسُولَ اللَّهِ أَيُّ الذَّنْبِ أَعْظَمُ قَالَ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنْ تَجْعَلَ لِلَّهِ نِدًّا وَهُوَ خَلَقَكَ قُلْتُ ثُمَّ أَيُّ قَالَ أَنْ تَقْتُلَ وَلَدَكَ خَشْيَةَ أَنْ يَأْكُلَ مَعَكَ قَالَ ثُمَّ أَيُّ قَالَ أَنْ تُزَانِيَ حَلِيلَةَ جَارِكَ وَأَنْزَلَ اللَّهُ تَصْدِيقَ قَوْلِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَالَّذِينَ لَا يَدْعُونَ مَعَ اللَّهِ إِلَٰهًا آخَرَ وَلَا يَقْتُلُونَ النَّفْسَ الَّتِي حَرَّمَ اللَّهُ إِلَّا بِالْحَقِّ وَلَا يَزْنُونَ
6001 صحيح البخاري كتاب الأدب باب قتل الولد خشية أن يأكل معه
86 صحيح مسلم كتاب الإيمان باب كون الشرك أقبح الذنوب وبيان أعظمها بعده