Musnad Ahmad 31:811

'Ali ibn 'Asim narrated to us, Sa'id ibn Iyas al-Jurayri narrated to us, from Abu Nadra, from Abu Sa'id al-Khudri, who said—I believe he attributed it to the Prophet, peace and blessings of Allah be upon him—he said: "If you come upon a garden, call out to its owner three times. If he answers you, [then ask permission]; otherwise, eat without causing damage. And if you come upon a shepherd, call out to him three times. If he answers you, [then ask permission]; otherwise, drink without causing damage." He said: And the Messenger of Allah, peace and blessings of Allah be upon him, said: "Hospitality is for three days, and whatever is beyond that is charity."