It was narrated that 'Abdullah (رضي الله عنه) said:

I said: O Messenger of Allah, which sin is the worst before Allah? He said: "Attributing a rival to Allah when He has created you." I said: Then what? He said: “Then killing your child for fear that he may share your food." - On one occasion 'Abdur-Rahman said: "for fear that he may eat with you"- I said: Then what? He said: "Then committing adultery with your neighbour's wife.” Then Allah revealed, confirming that: