Sulayman bin Yasar (may Allah have mercy on him) said: Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) came to Makkah while he was caliph, and the case of a man was brought before him who had freed a sa'ibah; that (sa'ibah) had killed a son of al-Sa'ib bin A'idh bin Abdullah bin Umar bin Makhzum by mistake. Al-Sa'ib asked Sayyiduna Umar Faruq (may Allah be pleased with him) for the blood money of his son. Sayyiduna Umar (may Allah be pleased with him) said: If he has wealth, he will pay the blood money for your son, whatever it amounts to. Al-Sa'ib said: And if he has no wealth? Sayyiduna Umar (may Allah be pleased with him) said: Then you will get nothing. Al-Sa'ib said: What if I kill him by mistake? He said: By Allah! You will pay (his blood money). Al-Sa'ib said: If he is killed, blood money is paid, but if he kills, no blood money is paid? Sayyiduna Umar (may Allah be pleased with him) said: Yes. Al-Sa'ib said: Then he is like the arqam (a venomous snake): if it is left alone, it devours, and if it is killed, it takes revenge. Sayyiduna Umar (may Allah be pleased with him) said: So it is. And he was not given anything.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الولاء / 21481