Sahih Muslim Book 7, Hadith Number 2781.

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.

Al-Qasim narrated from the Mother of the Believers (Hadrat ‘Aisha) that she said: Messenger of Allah, the people return (from Mecca) having done two worships (both Hajj and Umra), but I am coming back with one (only). whereupon he said: You should wait and when the period of menses is over, you should go to Tan’im and put on ihram and then meet us at such and such time (and I think he said tomorrow); and (the reward of this Umra) is for you equal to your hardship or your spending.