It was narrated that 'Abdullah said:

A man came to the Prophet (ﷺ) and said: Which sin is the worst? He said: "Attributing a rival to Allah when He has created you." He said: Then what? He said: "Then killing your child for fear that he may share your food.” He 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, not commit illegal sexual intercourse - and whoever does this shall receive the punishment.” (al-Furqan 25:68).