Marriage to Hazrat Ummu Kulthoom (radhiyallahu ‘anha):

After Hazrat Ruqayyah (radhiyallahu ‘anha) passed away, Hazrat ‘Uthmaan (radhiyallahu ‘anhu) married Hazrat Ummu Kulthoom (radhiyallahu ‘anha), another blessed daughter of Rasulullah (sallallahu ‘alaihi wasallam).

After the demise of Hazrat Ummu Kulthoom (radhiyallahu ‘anha), Rasulullah (sallallahu ‘alaihi wasallam) mentioned to Hazrat ‘Uthmaan (radhiyallahu ‘anhu), “If I had a third daughter (who was unmarried), I would have married her to you (as well).”

Hazrat ‘Ali (radhiyallahu ‘anhu) reports that he heard Rasulullah (sallallahu ‘alaihi wasallam) mention, “If I had forty daughters, I would have given them in marriage to Hazrat ‘Uthmaan (radhiyallahu ‘anhu), one after the other, until no daughter remained (i.e. after the demise of each one, I would have married him to the next daughter, until no daughters remained alive).”

(Usdul Ghaabah 3/216)