Musnad Ahmad 30:931

Abu Kamil narrated to us, Hammad narrated to us, from Muhammad ibn Ziyad, from Abu Hurairah, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) was brought food from someone other than his family, he would ask about it. If it was said: "It is a gift," he would eat. And if it was said: "It is charity," he would say: "Eat," and he would not eat.