(37) باب كَوْنِ الشِّرْكِ أَقْبَحَ الذُّنُوبِ وَبَيَانِ أَعْظَمِهَا بَعْدَهُ
'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.
Reference: Sahih Muslim 86 a
In-book reference: Book 1, Hadith 163
USC-MSA web (English) reference(deprecated numbering scheme): Book 1, Hadith 156