The hadith of Abu Juhayfah and Qays bin Abbad from Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) has already passed, concerning what he had (narrated) from the Prophet (peace and blessings be upon him), (written) in the scroll: "A Muslim is not to be killed in return for a disbeliever."
And in this there is an indication of the weakness of the (narration) that follows.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / Q15934
Abu al-Janub al-Asadi said: A Muslim man who had killed a dhimmi was brought to Ali (may Allah be pleased with him). He (may Allah be pleased with him) had ordered that he be killed when his brother came and said: I have forgiven his blood. Ali (may Allah be pleased with him) asked: Were you not forced into this? He said: No. Killing him will not bring my brother back. They gave me the blood money and I am content. Ali (may Allah be pleased with him) said: You know that whoever is under our protection, his blood and his blood money are like ours.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15934
Imam al-Daraqutni (may Allah have mercy on him) said: Abu al-Janub is da'if (weak). Imam al-Shafi'i (may Allah have mercy on him) said: Regarding the hadith of Abu Juhayfah (may Allah be pleased with him) from Ali (may Allah be pleased with him): how could it be that Ali (may Allah be pleased with him) would narrate something from the Prophet (peace and blessings be upon him) and then act contrary to it?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15935