It was narrated that 'Abdullah said: "I said: 'O Messenger of Allah, which sin is the most grievous?' He said: 'Setting up a rival to Allah while it is He that has created you.' I said: 'Then what?' He said: 'Killing your child for fear that he may eat with you.' I said: 'Then what?' He said: 'Committing adultery with your neighbor's wife.'"
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ وَاصِلٍ، عَنْ أَبِي وَائِلٍ، عَنْ عَمْرِو بْنِ شُرَحْبِيلَ، عَنْ عَبْدِ اللَّهِ، قَالَ قُلْتُ يَا رَسُولَ اللَّهِ أَىُّ الذَّنْبِ أَعْظَمُ قَالَ " أَنْ تَجْعَلَ لِلَّهِ نِدًّا وَهُوَ خَلَقَكَ " . قُلْتُ ثُمَّ مَاذَا قَالَ " أَنْ تَقْتُلَ وَلَدَكَ خَشْيَةَ أَنْ يَطْعَمَ مَعَكَ " . قُلْتُ ثُمَّ مَاذَا قَالَ " أَنْ تُزَانِيَ بِحَلِيلَةِ جَارِكَ " .
sunnah.com