Assalamoalaikum Brother,
There are various different versions of this narration, Among them are the following:
1) Rasulullah (sallallahu ‘alayhi wa sallam) said:
Marry ‘Uthman, If I had a third daughter I would have married him (to her) and I have only married him to my daughters through divine revelation (wahy).
(Al Mu’jamul Kabir, Hadith: 490, -vol. 11-)
2) Sayyiduna ‘Uthman (radiyallahu ‘anhu) said to Sayyiduna ‘Abdullah Ibn ‘Abbas (radiyallahu ‘anhuma) that Rasulullah (sallallahu ‘alayhi wa sallam) said to me when he got me married to his second daughter, “If I had ten (daughters), I would have married them to you one at a time, for I am pleased with you”.
(Al Mu’jamul Awsat, Hadith: 6112)
3) Sayyiduna Abu Hurayrah (radiyallahu ‘anhu) reports that Nabi (sallallahu ‘alayhi wa sallam) said:
…”If I had ten daughters, I would have married them to ‘Uthman and I only got him married to my daughters through divine revelation (wahy)”
(Al Mu’jamul Kabir, Hadith: 1063 -vol. 15-)
4) Nabi (sallallahu ‘alayhi wa sallam) said:
If I had forty daughters, I would have married them one after the other to ‘Uthman.
(Tarikh Ibn ‘Asakir, vol. 39 pg. 43)
The above narrations are considered weak by Islamic scholars, support each other sufficiently.
(Refer: Majma’uz Zawaid, vol. 5 pg. 264 and Al Bidayah Wan Nihayah of Hafiz Ibn Kathir, vol. 7 pg. 377)
And Allah Ta’ala knows best