Narrated Ibn 'Abbas (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) married Maimuna (radiallaahu `anhaa) while he was in the state of Ihram, (only the ceremonies of marriage were held).
This opinion, as discussed by scholars, was that of Ibn `Abbas (radiallaahu `anhu) only, who was most likely mistaken in this. Maimuna (radiallaahu `anhaa) herself says she did not get married in the state of Ihraam. See Abu Dawud: 1843 and Muslim: 1411.
Also, `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.However, there's an exception. Someone who has exited the first stage of Ihraam may get married [contract only], however, intercourse will only be permissible once second stage of Ihraam has been exited completely as well. [IslamQA]
[Sunan an-Nasa'i 2844, Sahih Muslim 1409]