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]"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / Q15822