Musannaf Ibn Abi Shayba 36:534

Waki’ narrated to us, from al-A’mash, from Ibrahim al-Taymi, from ‘Amr ibn Maymun, who said: I used to leave the first row out of reverence for ‘Umar, and I was in the second row on the day he was struck. He came and said, "The prayer, O servants of Allah! Align yourselves." He then led us in prayer, and Abu Lu’lu’ah stabbed him two or three times. ‘Umar was wearing a yellow garment, and he placed it over his chest. Then he leaned forward while saying, "And the command of Allah is a decree determined." He killed and stabbed twelve or thirteen people. The people rushed toward him, so he leaned upon his dagger and killed himself.