سنن النسائي

It was narrated from 'Ubaid bin 'Umair that : His father - who was one of the Companions of the Prophet (ﷺ) [SAW] - told him: "A man said: 'O Messenger of Allah, what are the major sins?' He said: 'They are seven; the most grievous of which are associating others with Allah, killing a soul unlawfully and fleeing (from the battlefield) on the day of the march.'" It is abridged.