Yahya related to me from Malik, from Sadaqa ibn Yasar, thatAbdullah ibn Umar said, "By Allah, I would rather do umra before hajjand sacrifice an animal than do umra after hajj in the month ofDhu'l-Hijja." (ID)
USC-MSA web (English) reference: Book 20, Hadith 62 (ID)
Arabic reference (ID): Book 20, Hadith 767 (ID)