Narrated `Abdullah:
I or somebody, asked Allah's Messenger (ﷺ) "Which is the biggest sin in the Sight of Allah?" He said, "That you set up a rival (in worship) to Allah though He Alone created you." I asked, "What is next?" He said, "Then, that you kill your son, being afraid that he may share your meals with you." I asked, "What is next?" He said, "That you commit illegal sexual intercourse with the wife of your neighbor." Then the following Verse was revealed to confirm the statement of Allah's Messenger (ﷺ): "Those who invoke not with Allah, any other god, nor kill life as Allah has forbidden except for just cause, nor commit illegal sexual intercourse." (25.68)
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 284
Arabic reference: Book 65, Hadith 4761