I asked Allah's Messenger (ﷺ) "What is the biggest sin in the sight of Allah?" He said, "To set up rivals unto Allah though He alone created you." I said, "In fact, that is a tremendous sin," and added, "What next?" He said, "To kill your son being afraid that he may share your food with you." I further asked, "What next?" He said, "To commit illegal sexual intercourse with the wife of your neighbor."
Oneness, Uniqueness of Allah (Tawheed)
Sahih al-Bukhari - Hadith 7520
The Greatest Sin: Shirk
The Prophet Muhammad (ﷺ) identifies associating partners with Allah (shirk) as the gravest sin. This constitutes setting up rivals or equals to Allah in worship, lordship, or attributes, despite His being the sole Creator. Shirk violates the fundamental principle of Tawheed (Allah's Oneness) and is the only sin Allah may not forgive if one dies upon it without repentance.
Killing Children Out of Poverty
This second major sin refers to the pre-Islamic Arabian practice of burying daughters alive due to fear of poverty or shame. It demonstrates extreme distrust in Allah's provision (rizq) and violates the sanctity of life Allah granted. The Prophet emphasized that sustenance is from Allah alone, and no one should transgress His boundaries out of worldly anxiety.
Adultery With Neighbor's Wife
This third grave sin combines two transgressions: illicit sexual relations (zina) and violation of neighbor's rights. In Islam, neighbors have special rights to protection and respect. Committing adultery with a neighbor's wife breaches multiple trusts - with Allah, with one's own soul, with the neighbor, and with the community. It destroys family structures and social harmony.
Scholarly Commentary
Imam al-Qurtubi notes this hadith establishes a hierarchy of sins, with shirk as the foundation of all disobedience. Ibn Hajar al-Asqalani explains these sins progress from rights of Allah (huquq Allah) to rights of children (huquq al-walad) to rights of society (huquq al-ijtima'). The sequence teaches that corrupt belief leads to corrupt actions against family and community.