Musnad Ahmad 25:1579

'Umar ibn 'Ubayd narrated to us, from 'Ata' ibn al-Sa'ib, who said: We were invited to a meal, and Sa'id ibn Jubayr and Miqsam, the freed slave of Ibn 'Abbas, were present. When the food was placed, Sa'id said: "Has the news of what was said regarding food reached all of you?" Miqsam said: "Narrate, O Abu 'Abd Allah, to those who have not heard it." He said: Ibn 'Abbas narrated to me, saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When food is placed, do not eat from its middle, for the blessing descends upon its middle. Eat from its edges, or its two edges."