Is this true?
Sayyiduna ‘Umar ibn Al Khattab married Ummu Kulthum, the daughter of ‘Ali (radiyallahu ‘anhu) [despite her young age] because of her lofty lineage.
Yes this is correct.
Imam ‘Abdur Razzaq and Imam Bayhaqi (rahimahumallah) have recorded this narration. Sayyiduna ‘Umar (radiyallahu ‘anhu) cited the virtue of her relationship to Nabi (sallallahu ‘alayhi wa sallam) as the motivating factor when he asked Sayyiduna ‘Ali (radiyallahu ‘anhu) for her hand in marriage.
(Musannaf ‘Abdur Razzaq, Hadith: 10354, As Sunanul Kubra, vol. 7 pg. 64)
Imam Bayhaqi (rahimahullah) has declared one version of the narration sound (mursalun hasanun)
And Allah Ta’ala Knows best.
Answered by: Moulana Suhail Motala
Approved by: Moulana Muhammad Abasoomar