Musnad Ahmad 27:620

He said: I found in my father’s book: Muhammad ibn Bakr narrated to us, Ibn Jurayj informed us, he said: ‘Ikrimah ibn Khalid said: I asked ‘Abdullah ibn ‘Umar about ‘Umrah before Hajj. Ibn ‘Umar said: "There is no harm upon anyone who performs ‘Umrah before he performs Hajj." ‘Ikrimah said: ‘Abdullah said: "The Prophet, may Allah bless him and grant him peace, performed ‘Umrah before he performed Hajj."