Imam al-Shafi'i (may Allah have mercy on him) said: "Allah, exalted be His praise, says: 'And do not kill the soul which Allah has forbidden (to be killed), except by right.' [Surah al-An'am: 151]
And He said: 'And those who do not call upon another god along with Allah, nor kill the soul which Allah has forbidden, except by right.' (the verse) [Surah al-Furqan: 68]"
Sayyiduna Abdullah (may Allah be pleased with him) said that a man came to him (peace and blessings be upon him) and asked about the major sins. He (peace and blessings be upon him) said: "To set up a partner with Allah though He is your Creator, to kill your child for fear of provision, and to commit fornication with your neighbour's wife." Then he (peace and blessings be upon him) recited this verse: '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]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / 15822