Sa'id bin Jubayr narrates from Ibn Abbas (may Allah be pleased with them both) regarding "We have certainly given his heir authority" [Surah al-Isra': 33]: that is, He made a way for him; and "so let him not exceed limits in killing" [Surah al-Isra': 33]: that is, he should not kill two in place of one. Imam al-Shafi'i (may Allah have mercy on him) said: "So let him not exceed limits in killing" [Surah al-Isra': 33] means that only the killer is to be killed in retaliation.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15887