Nahj al-Balāgha
Hadith 1443ثمّ قال: يَا بَنِي عَبْدِ الْمُطَّلِبِ، لَا أُلْفِيَنَّكُمْ تَخُوضُونَ دِمَاءَ الْمُسْلِمِينَ خَوْضاً، تَقُولُونَ: قُتِلَ أَمِيرُ الْمُؤْمِنِينَ. أَلَا لَا تَقْتُلُنَّ بِي إِلَّا قَاتِلِي.
Then he said: O’ sons of ‘Abd al-Muṭṭalib, certainly I do not wish to see you plunging harshly into the blood of Muslims shouting “Amīr al-mu’minīn has been killed.” Beware, do not kill on account of me except my killer.