Musannaf Ibn Abi Shayba 16:230

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from ‘Ata’ ibn al-Sa’ib, from Sa’id ibn Jubayr, from Ibn ‘Abbas, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When food is placed, eat from its edge and eat from its surroundings, and leave its top, for its top is the blessing."