Abdullah bin Ubayd bin Umayr (may Allah have mercy on him) said: When Sayyiduna Umar (may Allah be pleased with him) was stabbed, Sayyiduna Ubaydullah bin Umar (may Allah be pleased with them both) killed al-Hurmuzan. When Sayyiduna Umar (may Allah be pleased with him) was told of this, he asked: Why did you kill him? He replied: He killed my father. He asked: How so? He said: I saw him consulting with Abu Lu'lu'ah; it was he who ordered him to kill my father. Sayyiduna Umar (may Allah be pleased with him) said: I do not know what the matter is. Look, if I am martyred, ask Ubaydullah about this; if he produces a witness, then well and good; otherwise, take retaliation from him for al-Hurmuzan. When Sayyiduna Uthman (may Allah be pleased with him) became the ruler, it was said to him: Will you not carry out the bequest of Sayyiduna Umar (may Allah be pleased with him) regarding Ubaydullah? He asked: Who is the guardian of al-Hurmuzan? They said: You are, O Commander of the Believers! So Sayyiduna Uthman (may Allah be pleased with him) said: I have pardoned Ubaydullah bin Umar (may Allah be pleased with them both).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16083