Sahih Muslim Book 8, Hadith Number 3281.

Chapter: Mubrim (one who is in the state of Ihram) is forbidden to marry and the disapproval of giving the proposal (in this state).

‘Uthman (b. ‘Affan) reported it directly from Allah’s Apostle (May peace be upon him) that he said: A Muhrim should neither marry (in that state) nor make the proposal of marriage.