`Umrah before Hajj (Ahadith 1525 - 1526)

Volume 3, Book 27, Number 2:
Narrated Ibn Juraij:
Ikrima bin Khalid asked Ibn 'Umar (radiallaahu `anhu) about performing 'Umra before Hajj. Ibn 'Umar (radiallaahu `anhu) replied, "There is no harm in it." 'Ikrima said, "Ibn 'Umar (radiallaahu `anhu) also said, 'The Prophet (sallallaahu `alayhi wasallam) had performed 'Umra before performing Hajj.'"

Volume 3, Book 27, Number 3:
Narrated 'Ikrima bin Khalid:
"I asked Ibn 'Umar (radiallaahu `anhu) the same (as above)."
`Umrah may be done before/after/without Hajj. No restrictions on that, Alhamdulillah. But if one is going for Hajj, the type of Hajj they intended for must be kept in mind and they should act accordingly. For example, someone who's doing Tamattu` must do the `Umrah, come out of Ihraam, and then enter Ihraam for Hajj. He doesn't have an option to not do `Umrah before Hajj. Same for Qiraan, but the difference is that both `Umrah and Hajj are to be done in the same Ihraam (with one tawaaf and one sa`ee, of course). And for Ifraad, it's Hajj only. `Umrah after Ifraad/Qiran is something debatable. Go through this to know what I mean.
For more details on Hajj and its types and other issues related to it, search this blog as we've covered Kitaab-ul-Hajj Alhamdulillah. Or search IslamQA for whatever you can't find here.