Trustworthy and reliable Mohaddis Abdullah ibn Umar ibn Abaan al Ju'afi (d. 239 AH) said that my maternal uncle Hussain al Ju'afi asked me: Son! do you know why Usman رضیﷲ عنه was called "Zun Noorayn"?
I told him that I don't know about it.
He then said to me:
يجمع بين ابنتي نبي مذ خلق الله آدم إلى أن تقوم الساعة غير عثمان بن عفان فلذلك سمي ذا النورين
Allah عزوجل never let anyone marry two daughters of the same prophet from the creation of Adam عليه السلام till the judgement day except Saydna Usman عليه السلام. So this is why, he has given the title of "Zun Noorayn" (The keeper of two lights).
Prophet Mohammed صلی اللہ علیہ وسلم had 4 daughters Sayyidah Fatima, Sayyidah Ruqaiyah, Sayyidah Umme Kulthum, and Sayyidah Zainab.
Prophet Mohammad صلی اللہ علیہ وسلم married his two daughters Sayyidah Umme Kulthum and Sayyidah Ruqaiyah to Saydna Usman and he married his daughter Sayyidah Fatima to Saydna Ali.
[Sunan al Kubra lil Bayhaqi #13427, Maarifat As Sahabah la ibn Nu'aym #239 Tarikh Damishq 51/39] sanad Hasan by Shaikh Arshad Kamal
