It was narrated that ‘Abdullah said:

I said: O Messenger of Allah, which sin is worst?... And he narrated it, then he recited: "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. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace" (al-Furqan 25:68,69).