obedience to Him and to His messengers, and He has forbidden disobedience to Him and to His messengers. He has enjoined tawḥīd (pure monotheism) and sincerity.

Shirk—the gravest of sins and the core of Allah’s commands and prohibitions

He has forbidden associating anyone with Allah; therefore the greatest of good deeds is tawḥīd, and the greatest of sins is shirk.

Allah the Exalted says:
“Indeed, Allah does not forgive that partners be associated with Him, but He forgives anything short of that for whomever He wills.” (Q 4:48)

He also says:
“Yet among people are those who take others besides Allah as equals, loving them as they should love Allah; but those who believe are more intense in their love for Allah.” (Q 2:165)

In the two Ṣaḥīḥs it is reported that Ibn Masʿūd (may Allah be pleased with him) said: I asked, “O Messenger of Allah, which sin is the most serious?” He replied, “That you set up a rival to Allah while it is He who created you.” I asked, “Then which?” He said, “That you kill your child for fear that he will share your food.” I asked, “Then which?” He said, “That you commit adultery with your neighbor’s wife.” Thereafter Allah revealed in confirmation:
“And those who do not invoke another god alongside Allah, nor kill a soul that Allah has forbidden except by right, nor commit adultery—and whoever does that shall meet a penalty; it will be multiplied for him …” (Q 25:68–69)