Sahih al-Bukhari, Volume 6, Book 60, Hadith 26 (USC-MSA) ⓘ
Narrated Anas: The Prophet said, "The prescribed Law of Allah is the equality in punishment (i.e. Al-Qisas)." (In cases of murders, etc.)
حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ الأَنْصَارِيُّ، حَدَّثَنَا حُمَيْدٌ، أَنَّ أَنَسًا، حَدَّثَهُمْ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " كِتَابُ اللَّهِ الْقِصَاصُ " .
also referenced under: Book 65, Hadith 4499 (In-Book) & Hadith 4499 (DarusSalam)