Musnad Ahmad 26:872

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Wasil al-Ahdab, from Abu Wa'il, from Abdullah, who said: I asked the Messenger of Allah, peace and blessings of Allah be upon him: "Which sin is the greatest?" He said: "That you set up an equal to Allah, while He created you; and that you commit adultery with the wife of your neighbor; and that you kill your child out of fear that he will eat with you, or eat your food."