Sahih Muslim Book 1, Hadith Number 156.

Chapter : Polytheism is the most shameful of sins and the mentioning of the gravest sin next to it.

‘Abdullah reported: I asked the Messenger of Allah (may peace be upon him): 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.