Praise be to Allah.

Firstly: 

It is narrated in saheeh reports that the two angels whose task it is to question the deceased are black and blue, and one is called al-Munkar and the other is called al-Nakeer. 

Al-Tirmidhi narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “When the deceased – or one of you – is buried, there come to him two black and blue angels, one of whom is called al-Munkar and the other al-Nakeer. They say: ‘What did you say about this man?’ and he says what he used to say: ‘He is the slave of Allah and His Messenger. I bear witness that there is no god but Allah and that Muhammad is His slave and Messenger.’ They say: ‘We knew that you would say that.’ Then his grave is made spacious for him, seventy cubits by seventy, and it is illuminated for him. Then it is said to him: ‘Sleep,’ and he says: ‘May I go back to my family and tell them?’ They say: ‘Sleep like the bridegroom who will be woken by none but the dearest of his family to him,’ until Allah raises him from that resting-place of his.” 

But if he is a hypocrite he says: ‘I heard the people saying something so I said something like what they said. I do not know.’ They say: ‘We knew that you would say that.’ Then it is said to the earth: ‘Squeeze him.’ So it squeezes him until his ribs interlock, and he will continue to be tormented therein until Allah raises him from that resting-place of his.” 

This hadeeth was classed as hasan (sound) by al-Albani in Saheeh al-Tirmidhi. 

With regard to the hadeeth of ‘Umar  (may Allah be pleased with him) to which you refer, it was narrated via da’eef (weak) isnads (chains of narration), such as the mursal (one whose chain stops at before reaching the companions) report narrated by ‘Abd al-Razzaq in his Musannaf from ‘Amr ibn Dinar, according to which the Prophet (peace and blessings of Allah be upon him) said to ‘Umar (may Allah be pleased with him): “How will you deal, O ‘Umar, with the two testers in the grave, when they come to you digging the earth with their eyeteeth, stepping on their hair (because it is so long), their eyes like dazzling lightning and their voices like deafening thunder, with a whip so heavy that if the whole world were to gather together to lift it, they would not be able?” ‘Umar (may Allah be pleased with him) said: “I will be as I am today”. He said: “As you are today?” He said: “Then I shall take care of them for you, in sha Allah.” 

Al-‘Iraqi said in Takhreej al-Ihya: This was narrated like this, as a mursal report, by Ibn Abi’l-Dunya in Kitab al-Quboor, and its men are thiqat (trustworthy). Al-Bayhaqi said in al-I’tiqad: We have narrated it via a saheeh isnad in a mursal report from ‘Ata’ ibn Yasar. 

I say: Ibn Battah narrated it in a mawsool report (reaching the Prophet) in al-Ibanah, from the hadeeth of Ibn ‘Abbas (may Allah be pleased with him), and it was narrated by al-Bayhaqi in al-I’tiqad from the hadeeth of ‘Umar (may Allah be pleased with him), but he said it is ghareeb (strange) with this isnaad, which was narrated only by Mufaddal. Ahmad and Ibn Hibban also narrated from the hadeeth of ‘Abd-Allah ibn ‘Umar  (may Allah be pleased with him) that ‘Umar (may Allah be pleased with him) said: “Will our awareness be restored to us?” He said: “Yes, just as you are today.” ‘Umar  (may Allah be pleased with him) said: “Then I will throw a stone in his mouth.” end quote. 

Professor Fareej ibn Salih al-Bahlal said in his commentary on al-I’tiqad by al-Bayhaqi, after referring to the da’eef isnad (weak chain of narration): Perhaps by virtue of these isnads and corroborating reports its isnad may be hasan. End quote.