Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.033.

In it a pilgrim declares his intention to perform both Hajj and ‘Umrah together, and says when doing talbiyah, “O Allah, I answer your call to perform Hajj and ‘Umrah.” Such a pilgrim is required to remain in the state of ihram until he has performed all the rites of ‘Umrah and Hajj. Or, for this kind of ihram, a pilgrim may declare his intention to perform ‘Umrah and later, before making the Tawaf around Ka’bah, make his intention to perform Hajj as well.