Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: I asked the Messenger of Allah (peace and blessings be upon him): Which deed is the most excellent? He said: "Performing the prayer at its time." I said: Then what? He said: "Kindness to parents." I said: Then what? He said: "Jihad in the path of Allah." By Allah, had I asked him for more, he (peace and blessings be upon him) would have told me more. I said: Which sin is the gravest in the sight of Allah? He said: "That you set up a rival to Allah while it is He who created you." I said: Then which? He said: "That you kill your child for fear that it will eat with you." I said: Then which? He said: "That you commit adultery with your neighbour's wife." We did not remain long before Allah revealed this verse confirming it: "And those who do not call upon another god along with Allah, nor kill the soul which Allah has made sacred except by right, nor commit adultery; and whoever does that will meet a grievous penalty of sin" [Surah al-Furqan: 68].
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9784