The Gravest Sins

Bismillah wal Hamdulillah wa Salat u wa Salam Ala Rasul Allah

Narrated Abdullah : A man said, "O Allah's Messenger! What is the greatest sin with Allah?" The Prophet said, "To set up rivals unto Allah though He (Alone) created you." The man said, "What is next?" The Prophet said, "To kill your son lest he should share your food with you." The man said, "What is next?" The Prophet said, "To commit illegal sexual intercourse with the wife of your neighbour.” Then Allah revealed in confirmation of that:

“And those who invoke not any other ilah (god) along with Allah, nor kill such life 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 for him…” (V.25:68,69 )

[Al Bukhari]

(Book: 100 Ahadith about Islamic Manners by Darussalam Publications)