Was it authentic Umar (ra) ordered companions to kill those opposing appointment of khalif after 3 days and 2 nights?

In a Nutshell: The notion is authentic, but it was narrated in weak and authentic narrations as well. Background When Umar (ra) was stabbed and about to die, he recommended six of the senior companions for Muslims to choose one of them as a caliph after him. It was narrated that he said when Muslims agree on one of them, anyone will disagree has to be killed because his action will weaken the strength of Muslims. It was narrated that he said to Suhayb (ra): صَلِّ بِالنَّاسِ ثَلاثًا وَلْيَخْلُ هَؤُلاءِ الْقَوْمُ فِي بَيْتٍ فَإِذَا اجْتَمَعُوا عَلَى رَجُلٍ فَمَنْ خَالَفَهُمْ فَاضْرِبُوا رَأْسَهُ "Lead people in prayers for three days and these (six candidates and the mediators) should stay alone in a house. If they all elected a man of them, cut off the head of who will disagree." (Ibn Saad, al-Tabaqat al-Kubra, Vol. 3, p. 260, Tareekh al-Tabari, Vol. 4, p. 229, Abu Shabah, Tareekh al-Madinah, Vol. 3, p. 924, al-Khalal, al-Sunah, p. 278) There are a lot of narrations carries the same meaning, for example, it was narrated that Umar (ra) also said: وَيَتْبَعُ الْأقلُ الْأَكْثَرَ ، وَمِنْ تَأْمَرَ مِنْ غَيْرِ أَنْ يُؤَمَّرَ فَاُقْتُلُوهُ "The minority (of electors) have to follow (the choice of) the majority, and anyone who put himself in the leadership without permission, kill him." (Ibn Hajar, Fath al-Bari, Vol. 13, p. 196) This hard way has to be established because if the fitnah (trial) initiated, there would be a large number of killed Muslims. So, it has to be stopped from the beginning. The Authenticity of the Notion This notion was narrated in a number of weak and authentic narrations as well. The critics of this notion only criticise the narrations of Tarikh al-Tabari as its chain of narrators (isnad) contains an untrustworthy narrator, while the other narrations are at least hassan (less than sahih but accepted) especially the narration of ibn Saad in al-Tabaqat al-Kubra. Ibn Hajar, for instance, referred to the narration of ibn Saad as authentic: وَقَدْ أَخْرُجَ بْنُ سَعْدٍ بِسَنَدِ صَحِيحٍ مِنْ طَرِيقِ الزُّهْرِيِّ عَنْ سَالِمٍ عَنْ بْنُ عُمَر "Ibn Saad narrated with an authentic isnad (chain of narrators) on the authority of al-Zuhri from Salim from ibn Umar (ra)." (Ibn Hajar, Fath al-Bari, Vol. 13, p. 196) In a contemporary critical study focused on the narrations about the statements of Umar (ra) authored by Abd al-Salam Muhsin, all the narrations of the event are included and checked and he concluded for two narrations to be authentic (including one narration reported in the four books references above): وَسَنَدُهُ مُتَّصِلٌ وَرُجَّالُهُ ثِقَاتٌ، فَالْأثَرُ صَحِيحٌ ... وَرُجَّالُ إِسْنادِهِ عِنْدَ اِبْنِ سَعْدٍ ثِقَاتٍ "Its isnad is mutasil (unbroken) and its narrators are trustworthy, so the narration id authentic … (the author added regarding another narration) The narrator of its isnad in ibn Saad are trustworthy." (Abd al-Salam Muhsin, Dirasat Naqdiyah fi al-Marwiyaat al-Waridah fi Shakhsiyat Umar Ibn al-Khatab, pp. 1112-1116) Even Ibn Taymiyah who approached another narration that contains a fabricated part, he rightly only attacked the meaning others understood from it and referred to a hadith carries the same meaning above: بَلْ يَكُونُ قَدَّ أَمَرَ بِقَتْلِ مَنْ يَقْصِدُ الْفِتْنَةَ، كَمَا قَالَ النَّبِيُّ صَلَّى اللهُ عَلَيه وَسَلَّمَ: مِنْ جَاءَكُمْ وَأَمْرَكُمْ عَلَى رَجُلٍ وَاحِدٍ، يُرِيدُ أَنْ يُفَرِّقَ جَمَاعَتَكُمْ فَاِضْرِبُوا عُنُقَهُ بِالسَّيْفِ كَائِنًا مَنْ كَانَ "The order (of Umar) referred to those who intended the fitnah, as the Prophet (saw) said: who put himself forward (on authority) while you elected a man and he wants to depart you jama'ah, cut his head off by the sword regardless of his status." (Ibn Taymiyah, Minhaj al-Sunnah, Vol. 273) Marginal Issue One of the interesting parts of this narration is the fact that Muslims should not live without a Khilafah (caliphate) more than three days as Umar (ra) pointed out and as Talhah (ra) understood from his order. Talhah (ra) told them when they wanted to delay the election further than three days: لَا وَالَّذِي ذَهَبَ بِنَفْسِ عُمَرَ ؛ لَا أَزَيِّدكُمْ عَلَى الْأيَّامِ الثَّلاثَةِ الَّتِي أُمِرْتُم، ثُمَّ أَجْلِسُ فِي بَيْتِي؛ فَأَنْظُرُ مَا تَصْنَعُونَ "No, I swear by the one who took the soul of Umar (ra), I won't give you more than the three days you are entitled (to handle the issue on them) and I will stay in my house looking (or waiting) for what you do." What about Muslims now are living since 1924 without a caliphate and without unity between them. Conclusion The notion is authentic, but it was narrated in weak and authentic narrations as well. References Abu Shabah, Tareekh al-Madinah Abd al-Salam Muhsin, Dirasat Naqdiyah fi al-Marwiyaat al-Waridah fi Shakhsiyat Umar Ibn al-Khatab Al-Khalal, al-Sunah Ibn Hajar, Fath al-Bari Ibn Taymiyah, Minhaj al-Sunnah Ibn Saad, al-Tabaqat al-Kubra Tabari, Tarikh al-Rusul wa al-Muluk