Yahya related to me from Malik, from Nafi, from Abdullah ibnUmar, that Umar ibn al-Khattab said, "Keep your hajj separate fromyour umra. That way your hajj will be more complete. And your umrawill be more complete if you do it outside of the months of the hajj."
USC-MSA web (English) reference: Book 20, Hadith 68
Arabic reference: Book 20, Hadith 772