Bukhari Vol. 6 : No. 284

Narrated 'Abdullah (Radi-Allahu 'anhu):

I or somebody, asked Allah's Apostle (Sallallahu 'Alaihi Wa Sallam) "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 neighbour." Then
the following Verse was revealed to confirm the statement of Allah's
Apostle (Sallallahu 'Alaihi Wa Sallam):

"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)

Bukhari Vol. 6 : No. 284