Maliks Muwatta Book 20, Hadith Number 42.
Yahya related to me from Malik that he had heard some of the people of knowledge say, “If someone goes into ihram to do umra and then wants to go into ihram to do hajj as well, he can do so, as long as he has not done tawaf of the House and s’ay between Safa and Marwa. This is what Abdullah ibn ‘Umar did when he said, ‘If I am blocked from the House we shall do what we did when we were with the Messenger of Allah, may Allah bless him and grant him peace.’ He then turned to his companions and said, ‘It is the same either way. I call you to witness that I have decided in favour of hajj and umra together.'”
Malik said, “The companions of the Messenger of Allah, may Allah bless him and grant him peace, went into ihram to do umra in the year of the farewell hajj, and the Messenger of Allah, may Allah bless him and grant him peace, said to them, ‘Anyone that has a sacrificial animal with him should go into ihram to do hajj and umra together, and he should not come out of ihram until he has finished both.'”