Imam Bukhari and Imam Muslim (rahimahumallah) have recorded this narration.

(Sahih Bukhari, Hadith: 4477 and Sahih Muslim, Hadith: 86)

Translation

‘Abdullah ibn Mas’ud (radiyallahu ‘anhu) once asked Nabi (Sallallahu ‘alayhi wa sallam), ‘Which is the greatest sin in the eyes of Allah?’ He replied, ‘That you ascribe a partner to Allah whereas He created you.’ I said, ‘That is indeed a great sin.’ I asked, ‘Then what?’ He said, ‘That you kill your child fearing that he will share your food with you.’ I said, ‘Then what?’ He answered, ‘That you commit adultery with your neighbour’s wife.’

Also, some Hadiths warn us about the seven major sins, what are these seven sins?

Answer

The seven major sins are:

1) Ascribing partners with Allah (shirk)

2) Practicing magic.

3) Murdering an innocent soul.

4) Usurping the wealth of the orphan.

5) Consuming interest (riba)

6) Fleeing from the battlefield (unnecessarily)

7) Slandering innocent chaste believing women.

(Sahih Muslim, Hadith: 258)

And Allah Subhanahu Wa Ta’ala Knows best.