It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that Umar (may Allah be pleased with him) was stabbed at the pre-dawn time. He was stabbed by Abu Lu'lu'ah, who was the slave of al-Mughirah bin Shu'bah and was a Magian (Zoroastrian).
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 76