Musnad Ahmad 27:1269

Isma'il ibn Muhammad narrated to us, 'Abbad—meaning Ibn 'Abbad—narrated to us, 'Ubayd Allah ibn 'Umar narrated to me, from Nafi', from Ibn 'Umar, who said: "We entered into the state of Ihram with the Messenger of Allah (may Allah bless him and grant him peace) for Hajj, performing it exclusively (mufridan)."