It was narrated that 'Abdullah (رضي الله عنه) said:

I asked The Messenger of Allah (ﷺ): Which sin is the worst before Allah? He said: "Attributing a partner to Allah when He has created you.” I said: Then what? He said: "Then killing your child for fear that he may share your food.” I said: Then what? He said: "Then committing adultery with your neighbour's wife.” Then Allah revealed, confirming that: “And those who invoke not any other ilah (god) along with Allah, nor kill such person as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse - and whoever does this shall receive the punishment" (al-Furqan 25:68).