Aboo Lu'lu'ah was a slave for Mughira ibn Shu'ba and he used to make millstones and Al-Mughira used to to manipulate his labour by giving him four dirhams every day. Then Aboo Lu'lu'ah met Umar ibn Al-Khattab (to complain about his exhaustion) and addressed him; 'O commander of the faithful, Al-Mughira made heavier the burden of my livelihood so ask him to ease me."
Therefore Umar told him; 'Fear God and do good to your master.' Therefore the slave got angry and said; 'Your righteousness contained all other people [but not me!]' Therefore he made up his mind to kill him. He made a dagger with two heads and poisoned it and came to Al-Hormuzan (Persian King) and said; 'How do you see this?' Then he said; 'You do not hit anyone by it but you kill him!'
He said [Abi Rafi]; and Aboo Lu'lu'ah awaited for Umar, therefore he came to him at the fajr (morning prayers) until he stood behind him. And Umar, whenever the prayers commended, used to say; 'Establish your rows' and he said what he used to always say and when he said takbeer, Aboo Lu'lu'ah hit him in his shoulder and hit him in his side, then Umar fell down and he hit by his dagger thirteen men.
Among them seven were killed and Umar was carried and took to his house and people shouted until the sun was about to rise, then Abdul-Rahman Ibn Awf said, 'O people! The prayer! The prayer!' He said [Abi Rafi’]; Then they [people] rushed to the prayer, then Abdul-Rahman ibn Awf took the lead and prayed leading them with the shortest two Surah's of the Qur'aan.
And when he accomplished his prayer, they headed to Umar and he called for a drink to know the intensity of his wound. So wine was brought and he drank it, and it came out of his wound, and he did not know if it was wine or blood. Then he called for yoghurt, and he drank it and it came out of his wound then they said; 'O commander of the faithful, there is no harm [to you]!' He said; 'If being killed is a harm then I am killed."
Footnote: Narration is Saheeh (Authentic).