It was narrated that 'Abdullah said: "I said: 'O Messenger of Allah, which sin is most grievous?' He said: 'Setting up a rival to Allah while it is He that created you.' I said: 'Then what?' He said: 'Killing your child so that he will not eat with you.' I said: 'Then what?' He said: 'Committing adultery with your neighbor's wife.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا يَحْيَى، قَالَ حَدَّثَنَا سُفْيَانُ، قَالَ حَدَّثَنِي وَاصِلٌ، عَنْ أَبِي وَائِلٍ، عَنْ عَبْدِ اللَّهِ، قَالَ قُلْتُ يَا رَسُولَ اللَّهِ أَىُّ الذَّنْبِ أَعْظَمُ قَالَ " أَنْ تَجْعَلَ لِلَّهِ نِدًّا وَهُوَ خَلَقَكَ " . قُلْتُ ثُمَّ أَىٌّ قَالَ " أَنْ تَقْتُلَ وَلَدَكَ مِنْ أَجْلِ أَنْ يَطْعَمَ مَعَكَ " . قُلْتُ ثُمَّ أَىٌّ قَالَ " ثُمَّ أَنْ تُزَانِيَ بِحَلِيلَةِ جَارِكَ " .
sunnah.com