Musnad Abu Ya'la al-Mawsili 150:175

Abu Hisham Muhammad ibn Yazid ibn Rifa'ah narrated to us, Ishaq ibn Sulayman al-Razi narrated to us, Mu'awiyah narrated to us, from al-Zuhri, from 'Urwah, from 'Aishah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Bring near to us the blessed morning meal," meaning the Suhur, "and it may not have been anything but two dates." Al-Zuhri said: "The Suhur is a Sunnah."