In a Nutshell:Background
The hadith is not sahih as there is a missing narrator who was supposed to hear from the companion, but it could be supported by other shawahid (similar narration) prove the credibility of the notion.
It was narrated in the collection of Darmi that the Umar noted there is no Islam without jama'ah (Muslims gathering in a certain providence), there is not jama'ah without Imarah (leadership or Khilafah) and there is not Imarah without the obedience to the Leader of the Caliphate (when he enjoys good).
It was narrated he said:
إِنَّهُ لَا إِسْلَامَ إِلَّا بِجَمَاعَةٍ، وَلَا جَمَاعَةَ إِلَّا بِإِمَارَةٍ، وَلَا إِمَارَةَ إِلَّا بِطَاعَةٍ
"Indeed, there is no Islam without jama'ah, and there is not jama'ah without imarah, and there is no Imarah without obedience." (Darami 257)
This narration was reported in (Ibn Abd al-Bar al-Qurtubi, Jami al-Bayan, Vol. 1, p. 263, Ibn Hajar, Ithaf al-Maharah, Vol. 12, 116) The Authenticity
This narration was reported on the authority of Safwan ibn Rostom back to Tameem al-Dari (ra). The critics of this narration assign two illahs (defects) in the sand (chain of narrators) mentioned by the editor of Sunan al-Darmi Hussin al-Darani in the footnote of this hadith. These two illahs are: Safwan is majhool (unknown) and the hadith has defected also of an inqitaa (the one who heard from the companion is not mentioned in the sand, but only his student (Abd al-Rahman ibn Maysarah)). (See the editor's comment on the narration above)
But whilst the Dhahabi and others considered the narrator as Majhool (Dhahabi, al-Mizaan al-I'tidaal, Vol. 2, p. 316), he was accepted was noted by a more skilful scholar of hadith al-Bukhari who mentioned him in his book al-Tarikh and mentioned his teachers and said nothing of his jahalah. (Bukhari, al-Tarikh al-Kabeer, Vol. 4, p. 309)
But for inqitaa, it is true and the first narrator was not contemporary to the companion (ra). But there are many narrations (shawahid) support the exact wording of the hadith, as well as other ahadith, support the notion.Shawahid:
There is a shahid (a related narration of the same event) for this narrations (even if not sahih) that support one another as both of them were narrated by two different tareeq (chain of narrators).
The fifth-century muhadith ibn Abd al-Bar al-Qurtubi narrated Umar (ra) said:
لَا إِسْلامَ إلّا بِطَاعَةٍ، و لَا خَيْرَ إلّا فِي الْجَمَاعَةِ
"There is no Islam except with Obedience (that must be to the leader of a certain state) and there is no good except in the jama'ah." (Ibn Abd al-Bar al-Qurtubi, al-Tamheed, 10, p. 491)
In addition, this notion is well known in the Islamic classical scholarship referred to like the notion of dar al-Islam used by the Prophet (saw). So, if you have a Muslim Muslim group who require a ruler and a Muslim state.
The scholars generally argued a region can only be called Dar al-Islam where control and sovereignty belong to Muslim rulers. It is insufficient that Muslims live in that land, or they are allowed to perform some aspects of Islam with the permission of disbelievers. Similarly, non-Muslims residing in a land, conducting aspects of disbelief by the permission of Muslims has no bearing on the land being Dar al-Islam. This is because they do not have dominance - the status of a region hinges on dominance, not on presence or appearance.
So, Islam is where the Islamic government is established and vice versa. Read this answer
for more discussion on the notion.Conclusion
The hadith is not sahih as there is a missing narrator who was supposed to hear from the companion, but it could be supported by other shawahid (similar narration) prove the credibility of the notion.References
Bukhari, al-Tarikh al-Kabeer
Ibn Abd al-Bar al-Qurtubi, al-Tamheed
Ibn Abd al-Bar al-Qurtubi, Jami al-Bayan
Ibn Hajar, Ithaf al-Maharah