Sahih Muslim 86 a
'Abdullah reported:
I asked the Messenger of Allah (ﷺ): Which sin is the gravest in the eye of Allah? He (the Holy Prophet) replied: That you associate a partner with Allah (despite the fact) that He has created you. He (the reporter) said: I told him (the, Holy Prophet): Verily it is indeed grave. He (the reporter) said: I asked him what the next (gravest sin) was. He (the Holy Prophet) replied: That you kill your child out of fear that he shall join you in food. He (the reporter) said: I asked (him) what the next (gravest sin) was. He (the Holy Prophet) observed: Then (the next gravest sin) is that you commit adultery with the wife of your neighbour.
حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ، وَإِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ إِسْحَاقُ أَخْبَرَنَا جَرِيرٌ، وَقَالَ، عُثْمَانُ حَدَّثَنَا جَرِيرٌ، عَنْ مَنْصُورٍ، عَنْ أَبِي وَائِلٍ، عَنْ عَمْرِو بْنِ شُرَحْبِيلَ، عَنْ عَبْدِ اللَّهِ، قَالَ سَأَلْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم أَىُّ الذَّنْبِ أَعْظَمُ عِنْدَ اللَّهِ قَالَ ‏"‏ أَنْ تَجْعَلَ لِلَّهِ نِدًّا وَهُوَ خَلَقَكَ ‏"‏ ‏.‏ قَالَ قُلْتُ لَهُ إِنَّ ذَلِكَ لَعَظِيمٌ ‏.‏ قَالَ قُلْتُ ثُمَّ أَىٌّ قَالَ ‏"‏ ثُمَّ أَنْ تَقْتُلَ وَلَدَكَ مَخَافَةَ أَنْ يَطْعَمَ مَعَكَ ‏"‏ ‏.‏ قَالَ قُلْتُ ثُمَّ أَىٌّ قَالَ ‏"‏ ثُمَّ أَنْ تُزَانِيَ حَلِيلَةَ جَارِكَ ‏"‏ ‏.‏