Sahih Muslim Book 7, Hadith Number 2780.

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.

This Hadith has been narrated by Yahyi through the same chain of transmitters.