Sahih Muslim Book 7, Hadith Number 2775.

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 reported that the Messenger of Allah (may peace be upon him) entered into the state of Ihram for Hajj Ifrid.