Sahih Muslim Book 7, Hadith Number 2769.

Chapter : Types of Ihram- It is permissible to enter into the state of Ihram singularly for the sake of Hajj or for tamattu’ bi’l-‘umra ila’l-Hajj or for both hajj and ‘umra together.

‘Aisha (Allah be pleased with her) said: We set out with the Messenger of Allah (may peace be upon him) just at the appearance of the new moon of Dhu’l-Hijja. We had no other intention but that of performing the Hajj, whereupon the Messenger of Allah (may peace be upon him) said: He who among you intends to put on Ihram for ‘Umra should do so for ‘Umra. The rest of the Hadith is the same.