Umar ibn Al-Khattab (رضي الله عنه) was the second khalifa, and one of the ten companions promised Jannah in their lifetimes.
'Umar (رضي الله عنه) used to be a soldier–one of the best. But when he became the khalifah, the other companions refused to let him fight–he disagreed with them, but in the end, he stayed back in Medinah while the battles waged. Yet he always made one du'a, publicly and privately, throughout his khilafah: "Allahumma ruziqniy shahaadah," "O Allah, provide me with martyrdom."
People told him he was crazy–he was the Khalifah, of all people, and in a city full of Muslims! And, being the Khalifa, they didn't allow him to fight. But Subhanallah, Allah knew his sincerity, and answered his du'a.
While he was in Medina, there was a Majuwsi (fire-worshiper) who was a slave, a man by the name of Abu Lu' Lu'. Abu Lu' Lu' was an expert blacksmith, and a slave, captured during the conquest of Nahawand. He was in Medina, making things for the Muslims. His slave-master, though–a Muslim–would take a lot of his earnings.
So knowing the reputation of 'Umar ibn Al-Khattab (رضي الله عنه) for justice, he went to 'Umar and told him: "Ask my master to reduce what he's taking from me."
And 'Umar (رضي الله عنه) told him: "be patient."
Now, Abu Lu' Lu' got mad–furious! And he went away furious.
Meanwhile, 'Umar (رضي الله عنه) went to Abu Lu' Lu's master, and told him to reduce what he was taking from Abu Lu' Lu', which he agreed.
Unknown, Abu Lu' Lu' stewed in his rage. Being a master blacksmith, he crafted a knife–a special knife, made of two curving blades, made out of stone. He bought poison, and soaked his knife in it. He asked the person he bought the poison from, "if I cut someone with this, is there any chance they will survive?" and the person said "no."
Now, at that time, 'Umar (رضي الله عنه) would lead salaah in the masjid. And this was well-known. So one day, at Fajr, Abu Lu' Lu' snuck into the masjid. After salaah started, while 'Umar was praying, he jumped out and stabbed him.
One companion narrates, that 'Umar said: "The dog has eaten 'Umar."
Abu Lu' Lu' turned to make his escape; but the Muslims prayed so close to each other, he couldn't escape–so he stabbed his way out. He killed several companions (almost a dozen), until one of them threw a cloth on top of him and tackled him to the floor. He then commited suicide.
Now, imagine: Fajr in the masjid, ,and the Khalifa is lying on the floor in a pool of blood. One narration says that even then, at that time, 'Umar said "finish the salaah, finish the salaah"–because sunrise was coming. Subhanallah, priorities!
They knew 'Umar (رضي الله عنه) was going to die–he would eat food, and it would come out of his wounds. And when he realized who stabbed him, he said, why did the man stab him? He had gone to ask Abu Lu' Lu's slave master to lower his wages.
He sent a messenger to 'Aishah (رضي الله عنها). Abu Bakr (رضي الله عنه) and the Prophet (صلى الله عليه و سلم) were buried in the Prophet's house, and there was space for one more grave. So he sent a messenger to 'Aisha, and told him to ask her if he could have that spot.
'Aishah (رضي الله عنها) narrates: "I was thinking of saving that spot for myself, but …" and she gave it to him. When the messenger returned, 'Umar (رضي الله عنه) told him "go again, and this time, tell her 'Umar ibn Al-Khattab is asking, not Ameer al-Mu'mineen." Subhanallah! Even at the time of his death, he was worried that he was using his position to get gains for himself.
So the messenger went back, and 'Aishah (رضي الله عنها) said "yes, give it to him."
So he died (رضي الله عنه), and they buried him with the Prophet (صلى الله عليه و سلم) and Abu Bakr (رضي الله عنه).
And so, in the most unexpected time and place–the Khalifa praying Fajr in the masjid, surrounded by Muslims–Allah answered 'Umar's (رضي الله عنه) du'a and made him shaheed.