Sahih Muslim Book 8, Hadith Number 3280.

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 that Allah’s Messenger (May peace be upon him) had said: A Muhrim should neither marry himself, nor should he be got married to anyone, nor should he make the proposal of marriage.