Volume 3, Book 27, Number 20:
Narrated 'Amr bin Dinar:
We asked Ibn 'Umar (radiallaahu `anhu) whether a man who had performed the Tawaf of the Ka'ba but had not performed the Tawaf between As-Safa and Al-Marwa yet, was permitted to have sexual relation with his wife. He replied, "The Prophet (sallallaahu `alayhi wasallam) arrived (at Mecca) and circumambulated the Ka'ba seven times and then offered a two Rak'at prayer behind Maqam-lbrahim and then performed the going (Tawaf) between As-Safa and Al-Marwa (seven times) (and verily, in Allah's Apostle (sallallaahu `alayhi wasallam) you have a good example." And we asked Jabir bin 'Abdullah (radiallaahu `anhu) (the same question) and he replied, "He should not go near her till he has finished the going (Tawaf) between As-Safa and Al-Marwa."
Volume 3, Book 27, Number 21:
Narrated Abu Musa Al-Ashari (radiallaahu `anhu):
I came to the Prophet (sallallaahu `alayhi wasallam) at Al-Batha' while his camel was kneeling down and he asked me, "Have you intended to perform the Hajj?" I replied in the affirmative. He asked me, 'With what intention have you assumed Ihram?" I replied, "I have assumed Ihram with the same intention as that of the Prophet (sallallaahu `alayhi wasallam). He said, "You have done well. Perform the Tawaf of the Ka'ba and (the Sai) between As-safa and Al-Marwa and then finish the Ihram." So, I performed the Tawaf around the Ka'ba and the Sai) between As-Safa and Al-Marwa and then went to a woman of the tribe of Qais who cleaned my head from lice. Later I assumed the Ihram for Hajj. I used to give the verdict of doing the same till the caliphate of 'Umar (radiallaahu `anhu) who said, "If you follow the Holy Book then it orders you to remain in the state of Ihram till you finish from Hajj, if you follow the Prophet (sallallaahu `alayhi wasallam) then he did not finish his Ihram till the Hadi (sacrifice) had reached its place of slaughtering (Hajj-al-Qiran)."
Volume 3, Book 27, Number 22:
Abdullah the slave of Asma bint Abu Bakr (radiallaahu `anhaa), told me that he used to hear Asma' (radiallaahu `anhaa), whenever she passed by Al-Hajun, saying, "May Allah bless His Apostle Muhammad (sallallaahu `alayhi wasallam). Once we dismounted here with him, and at that time we were traveling with light luggage; we had a few riding animals and a little food ration. I, my sister, 'Aisha (radiallaahu `anhaa), Az-Zubair (radiallaahu `anhu) and such and such persons performed 'Umra, and when we had passed our hands over the Ka'ba (i.e. performed Tawaf round the Ka'ba and between As-Safa and Al-Marwa) we finished our lhram. Later on we assumed Ihram for Hajj the same evening."
As discussed before, Ihraam for `Umrah is the same as Ihraam for Hajj. A person may open his Ihraam [of `Umrah] once he's done with Tawaaf, Sa`ee and cutting/shaving hair.
More on Ihraam:
Ihraam means having the intention of starting the rituals of Hajj or ‘Umrah.
When the pilgrim wants to enter ihraam, he should change his clothes and do ghusl like the ghusl done to cleanse oneself of janaabah (impurity following sexual activity). Then he should apply whatever perfume is available, musk or whatever, to his head and beard. It does not matter if traces of that perfume remain after he enters ihraam, because of the hadeeth narrated in al-Saheehayn from ‘Aa’ishah (may Allaah be pleased with her) who said: When the Prophet (peace and blessings of Allaah be upon him) wanted to enter ihraam, he would perfume himself with the best perfume he could find, then I would see shining traces of that musk on his head and beard after that.” Narrated by al-Bukhaari, 271; Muslim, 1190.
Doing ghusl when entering ihraam is Sunnah for both men and women, even women who are bleeding following childbirth or menstruating, because the Prophet (peace and blessings of Allaah be upon him) commanded Asma’ bint ‘Umays when she was bleeding following childbirth to do ghusl when she entered ihraam, and to bandage her private parts and enter ihraam. Narrated by Muslim, 1209. Then after doing ghusl and putting on perfume, the pilgrim should put on the ihraam garments and – apart from women who are bleeding following childbirth or menstruating – pray the obligatory prayer if that is at the time of an obligatory prayer, otherwise they should pray two rak’ahs with the intention of the Sunnah prayer of wudoo’. When the pilgrim has finished praying he should face the qiblah and enter ihraam. He may delay ihraam until he has boarded his means of transportation, and is ready to move off, but he should enter ihraam before he leaves the meeqaat for Makkah.
Taken from: http://ilookilisten.wordpress.com/2013/05/05/umrah-ihraam-ahadith-1543-1545/