Hadith 39
Chapter 20: Hajj - كتاب الحج
Yahya related to me from Malik that he had heard the people of knowledge say, "If someone goes into ihram to do hajj on its own, he cannot then go into ihram to do umra.'' Malik said, "This is what I have found the people of knowledge in our city doing."
Reference Muwatta Malik 39
USC-MSA web (English) reference : Book 20, Hadith 39