Musnad Ahmad 30:3246

Bahz narrated to us, he said: Hammad ibn Salamah narrated to us, from Muhammad—meaning Ibn Ziyad—from Abu Hurairah: "That the Prophet, peace and blessings of Allah be upon him, when food was brought to him, he would ask about it. If it was charity, he would not eat it, and if it was a gift, he would eat it."