Sunan an-Nasa'i 4015

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