Musnad Ahmad 27:797

Waki' and 'Abd al-Rahman narrated to us, from Sufyan, from Jabalah ibn Suhaym, from Ibn 'Umar, who said—'Abd al-Rahman said: I heard Ibn 'Umar say: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade a man from pairing two dates together [when eating] until he asks permission from his companions."