Musnad Abu Ya'la al-Mawsili 154:65

Ahmad ibn Ibrahim al-Dawraqi narrated to us, Sa'id ibn 'Amir narrated to us, Habib ibn al-Shahid narrated to us, from Bakr ibn 'Abd Allah, from Anas: That the Messenger of Allah (ﷺ) entered into the state of ihram for both of them [Hajj and 'Umrah] together. Bakr said: I met Ibn 'Umar and informed him of the statement of Anas. Ibn 'Umar said: "He only entered into the state of ihram for Hajj." So I returned to Anas and informed him of the statement of Ibn 'Umar. He said: "Do you consider us to be nothing but children?" Or he said: "As if we were children." (Sa'id was in doubt).