The marriage of the second Khalīfah to Umm Kulthum, the daughter of Imam Alī (as), has been recorded in many narrations both in Shīʿah and Sunni books. The narrations regarding this event differ from one another; both the narrators and content of the traditions have been subject to scrutiny and debate. It can be concluded, in accordance to the common elements amongst the Shīʿah and Sunni accounts that the second Khalīfah made a proposal for Umm Kulthum through Imam Alī(as), he [even] threatened Imam Alī (as), nonetheless Imam Alī (as) made an excuse to reject the offer [again], Umar repeated his threats and on different occasions he continued proposing [even though Imam Alī would always disapprove].
In the end, Imam Alī(as) assigned the responsibility of approving any wedding proposals for Umm Kulthum to his uncle, Abbas ibn Abdal-Muṭṭalib. Therefore it was through his (Abbass) permission that the Nikāḥ took place and Umm Kulthum moved into Umars house. After Umars death, Imam Alī(as) brought her back to his own home.
This is the complete account as it appears in both Shīʿah and Sunni sources, however there is no reliable evidence that they consummated the marriage, or that Umm Kulthum bore any children from this marriage. Nonetheless, Shīʿah and Sunni scholars have used these traditions to draw conclusions and give their opinions regarding the event.
Nobakhti, one of the early Shīʿah scholars living in the time of the minor occultation has said: Umm Kulthum was a minor when Umar married her, and before she came of age (baligh), Umar left this world.
Similarly, Razqāni Mālikī, a Sunni Scholar (d. 1122 AH) says,Umm Kulthum was the wife of Umar ibnKhaṭṭāb, [though] before she came of age (baligh), Umar left this world.
 Tahdhib al-Tahdhib, vol. 1, pg. 44.
 Tabaqat Ibn Sad, vol. 3, pg. 259.
 Kāfī, vol. 5, pg. 246.
 Ibid, vol. 6, pg. 115.
 Bihar al-Anwar, vol. 42, pg. 91.
 Sharh al-Mawahib al-Daniyyah, vol.7, pg. 9.