for them the worst of what they did and grant them their reward according to the best of what they used to do.”
[Surah al-Zumar 32–35]

The ranking of major sins is established in the Quran and Sunnah. Thus, the two Sahih collections report that ʿAbdullāh ibn Masʿūd said:

“I asked, ‘O Messenger, which sin is greatest?’ He replied, ‘To set up a rival to Allah, although He created you.’ I asked, ‘Then which?’ He replied, ‘To kill your child for fear that he will share your food.’ I asked, ‘Then which?’ He replied, ‘To commit adultery with your neighbour’s wife.’”

This is confirmed in the Book of Allah:

“And those who do not invoke another god besides Allah, nor kill a soul that Allah has rightfully made inviolable, nor commit adultery.”
[al-Furqān 68]

For this reason, the jurists say that the greatest major sin is disbelief, followed by taking a life without right, then adultery. However, the Prophet, peace and blessings be upon him, mentioned to Ibn Masʿūd the gravest form of each category. The gravest form of disbelief is to set up a rival to Allah, unlike the disbelief of a person of the Book who is not a polytheist, which is less grave. The gravest form of killing is to kill your own child, and the gravest form of adultery is adultery with your neighbour’s wife.