Sahih muslim:86a Chapter: Explaining Al-Iman (Faith), Al-Islam, and Al-Ihsan, and The Obligations of Al-Iman With Affirmation of The Qadar of Allah, Glorious And Most High is He. And Explaining The Evidence For Declaring One’s Innocence Of One Who Does Not Believe In Al-Qadar, And Having A Harsh View Of His Case Book 1, Hadith 163

‘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.