It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he said: I said: O Messenger of Allah, which deed is the most excellent? He said: "Establishing the prayer at its time, kindness to parents, and jihad in the path of Allah."
I said: Which deed is the worst? He said: "That you set up a rival to your Creator, that you kill your child for fear that it will eat with you, or that you commit adultery with your neighbour's wife." And concerning this the Qur'an was revealed.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9775