Abd-Allaah ibn Masood (RAA) said: I asked the Messenger of Allah (SAW), Which sin is worst in the sight of Allah? He said, To make any rival to Allah, when He has created you. I asked, Then what? He said, To kill your child for fear that he will eat with you. I asked, Then what? He said, To commit zinaa with the wife of your neighbour." (Reported in Sahih Al-Bukhaari, Hadith #492 and In Shaih Muslim, Hadith #90).