It was narrated that 'Abdullah said:
 I asked the Messenger of Allah [SAW], which sin is most grievous?  He said:  Shirk, setting up a rival to Allah, committing adultery with your neighbor's wife, and killing your child for fear of poverty, and that he may eat with you.  Then 'Abdullah recited the Verse:  And those who invoke not any other Ilah (god) along with Allah.