Sayyiduna Abdullah (may Allah be pleased with him) said that a man came to the Prophet (peace and blessings be upon him) and asked: O Messenger of Allah, which sin is greatest in the sight of Allah? He (peace and blessings be upon him) answered: "To associate partners with Allah though He is your Creator." He asked: Then which? He said: "To kill your child for fear of poverty." He asked: Then which? He said: "To commit fornication with your neighbour's wife." Then Allah revealed in confirmation of it: 'And those who do not call upon another god along with Allah, nor kill the soul which Allah has forbidden, except by right, nor commit fornication; and whoever does that will meet the penalty.' [Surah al-Furqan: 68]
Imam al-Shafi'i (may Allah have mercy on him) said: Allah Almighty says: 'Whoever kills a soul other than for a soul or for corruption in the land, it is as if he has killed all of mankind.' [Surah al-Ma'idah: 32] and 'And recite to them the story of the two sons of Adam in truth' up to His words 'and he became one of the losers.' [Surah al-Ma'idah: 27-30]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / 15823