Musnad Ahmad 31:171

Yazid narrated to us, [he said] al-Jariri informed us, from Abu Nadra, from Abu Sa’id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, who said: "If you come upon a shepherd of camels, then call out: 'O shepherd of the camels,' three times. If he answers you, [then do so], and if not, then milk and drink without causing damage. And if you come upon a garden wall, then call out: 'O owner of the wall,' three times. If he answers you, [then do so], and if not, then eat." And the Messenger of Allah, peace and blessings of Allah be upon him, said: "Hospitality is three days, and whatever is beyond that is charity."